diff --git a/apps/api/lib/validations/destination-calendar.ts b/apps/api/lib/validations/destination-calendar.ts
index 371ae5ad51..15d1d8672c 100644
--- a/apps/api/lib/validations/destination-calendar.ts
+++ b/apps/api/lib/validations/destination-calendar.ts
@@ -3,7 +3,6 @@ import { z } from "zod";
 import { _DestinationCalendarModel as DestinationCalendar } from "@calcom/prisma/zod";
 
 export const schemaDestinationCalendarBaseBodyParams = DestinationCalendar.pick({
-  credentialId: true,
   integration: true,
   externalId: true,
   eventTypeId: true,
@@ -15,7 +14,6 @@ const schemaDestinationCalendarCreateParams = z
   .object({
     integration: z.string(),
     externalId: z.string(),
-    credentialId: z.number(),
     eventTypeId: z.number().optional(),
     bookingId: z.number().optional(),
     userId: z.number().optional(),
@@ -47,5 +45,4 @@ export const schemaDestinationCalendarReadPublic = DestinationCalendar.pick({
   eventTypeId: true,
   bookingId: true,
   userId: true,
-  credentialId: true,
 });
diff --git a/apps/api/pages/api/destination-calendars/[id]/_patch.ts b/apps/api/pages/api/destination-calendars/[id]/_patch.ts
index 7b5735f22c..0ea5b23598 100644
--- a/apps/api/pages/api/destination-calendars/[id]/_patch.ts
+++ b/apps/api/pages/api/destination-calendars/[id]/_patch.ts
@@ -1,6 +1,12 @@
+import type { Prisma } from "@prisma/client";
 import type { NextApiRequest } from "next";
+import type { z } from "zod";
 
+import { getCalendarCredentials, getConnectedCalendars } from "@calcom/core/CalendarManager";
+import { HttpError } from "@calcom/lib/http-error";
 import { defaultResponder } from "@calcom/lib/server";
+import type { PrismaClient } from "@calcom/prisma";
+import { credentialForCalendarServiceSelect } from "@calcom/prisma/selects/credential";
 
 import {
   schemaDestinationCalendarEditBodyParams,
@@ -56,16 +62,251 @@ import { schemaQueryIdParseInt } from "~/lib/validations/shared/queryIdTransform
  *       404:
  *        description: Destination calendar not found
  */
+type DestinationCalendarType = {
+  userId?: number | null;
+  eventTypeId?: number | null;
+  credentialId: number | null;
+};
+
+type UserCredentialType = {
+  id: number;
+  appId: string | null;
+  type: string;
+  userId: number | null;
+  user: {
+    email: string;
+  } | null;
+  teamId: number | null;
+  key: Prisma.JsonValue;
+  invalid: boolean | null;
+};
+
 export async function patchHandler(req: NextApiRequest) {
-  const { prisma, query, body } = req;
+  const { userId, isAdmin, prisma, query, body } = req;
   const { id } = schemaQueryIdParseInt.parse(query);
   const parsedBody = schemaDestinationCalendarEditBodyParams.parse(body);
+  const assignedUserId = isAdmin ? parsedBody.userId || userId : userId;
 
+  validateIntegrationInput(parsedBody);
+  const destinationCalendarObject: DestinationCalendarType = await getDestinationCalendar(id, prisma);
+  await validateRequestAndOwnership({ destinationCalendarObject, parsedBody, assignedUserId, prisma });
+
+  const userCredentials = await getUserCredentials({
+    credentialId: destinationCalendarObject.credentialId,
+    userId: assignedUserId,
+    prisma,
+  });
+  const credentialId = await verifyCredentialsAndGetId({
+    parsedBody,
+    userCredentials,
+    currentCredentialId: destinationCalendarObject.credentialId,
+  });
+  // If the user has passed eventTypeId, we need to remove userId from the update data to make sure we don't link it to user as well
+  if (parsedBody.eventTypeId) parsedBody.userId = undefined;
   const destinationCalendar = await prisma.destinationCalendar.update({
     where: { id },
-    data: parsedBody,
+    data: { ...parsedBody, credentialId },
   });
   return { destinationCalendar: schemaDestinationCalendarReadPublic.parse(destinationCalendar) };
 }
 
+/**
+ * Retrieves user credentials associated with a given credential ID and user ID and validates if the credentials belong to this user
+ *
+ * @param credentialId - The ID of the credential to fetch. If not provided, an error is thrown.
+ * @param userId - The user ID against which the credentials need to be verified.
+ * @param prisma - An instance of PrismaClient for database operations.
+ *
+ * @returns - An array containing the matching user credentials.
+ *
+ * @throws HttpError - If `credentialId` is not provided or no associated credentials are found in the database.
+ */
+async function getUserCredentials({
+  credentialId,
+  userId,
+  prisma,
+}: {
+  credentialId: number | null;
+  userId: number;
+  prisma: PrismaClient;
+}) {
+  if (!credentialId) {
+    throw new HttpError({
+      statusCode: 404,
+      message: `Destination calendar missing credential id`,
+    });
+  }
+  const userCredentials = await prisma.credential.findMany({
+    where: { id: credentialId, userId },
+    select: credentialForCalendarServiceSelect,
+  });
+
+  if (!userCredentials || userCredentials.length === 0) {
+    throw new HttpError({
+      statusCode: 400,
+      message: `Bad request, no associated credentials found`,
+    });
+  }
+  return userCredentials;
+}
+
+/**
+ * Verifies the provided credentials and retrieves the associated credential ID.
+ *
+ * This function checks if the `integration` and `externalId` properties from the parsed body are present.
+ * If both properties exist, it fetches the connected calendar credentials using the provided user credentials
+ * and checks for a matching external ID and integration from the list of connected calendars.
+ *
+ * If a match is found, it updates the `credentialId` with the one from the connected calendar.
+ * Otherwise, it throws an HTTP error with a 400 status indicating an invalid credential ID.
+ *
+ * If the parsed body does not contain the necessary properties, the function
+ * returns the `credentialId` from the destination calendar object.
+ *
+ * @param parsedBody - The parsed body from the incoming request, validated against a predefined schema.
+ *                     Checked if it contain properties like `integration` and `externalId`.
+ * @param userCredentials - An array of user credentials used to fetch the connected calendar credentials.
+ * @param destinationCalendarObject - An object representing the destination calendar. Primarily used
+ *                                    to fetch the default `credentialId`.
+ *
+ * @returns - The verified `credentialId` either from the matched connected calendar in case of updating the destination calendar,
+ *            or the provided destination calendar object in other cases.
+ *
+ * @throws HttpError - If no matching connected calendar is found for the given `integration` and `externalId`.
+ */
+async function verifyCredentialsAndGetId({
+  parsedBody,
+  userCredentials,
+  currentCredentialId,
+}: {
+  parsedBody: z.infer<typeof schemaDestinationCalendarEditBodyParams>;
+  userCredentials: UserCredentialType[];
+  currentCredentialId: number | null;
+}) {
+  if (parsedBody.integration && parsedBody.externalId) {
+    const calendarCredentials = getCalendarCredentials(userCredentials);
+
+    const { connectedCalendars } = await getConnectedCalendars(
+      calendarCredentials,
+      [],
+      parsedBody.externalId
+    );
+    const eligibleCalendars = connectedCalendars[0]?.calendars?.filter((calendar) => !calendar.readOnly);
+    const calendar = eligibleCalendars?.find(
+      (c) => c.externalId === parsedBody.externalId && c.integration === parsedBody.integration
+    );
+
+    if (!calendar?.credentialId)
+      throw new HttpError({
+        statusCode: 400,
+        message: "Bad request, credential id invalid",
+      });
+    return calendar?.credentialId;
+  }
+  return currentCredentialId;
+}
+
+/**
+ * Validates the request for updating a destination calendar.
+ *
+ * This function checks the validity of the provided eventTypeId against the existing destination calendar object
+ * in the sense that if the destination calendar is not linked to an event type, the eventTypeId can not be provided.
+ *
+ * It also ensures that the eventTypeId, if provided, belongs to the assigned user.
+ *
+ * @param destinationCalendarObject - An object representing the destination calendar.
+ * @param parsedBody - The parsed body from the incoming request, validated against a predefined schema.
+ * @param assignedUserId - The user ID assigned for the operation, which might be an admin or a regular user.
+ * @param prisma - An instance of PrismaClient for database operations.
+ *
+ * @throws HttpError - If the validation fails or inconsistencies are detected in the request data.
+ */
+async function validateRequestAndOwnership({
+  destinationCalendarObject,
+  parsedBody,
+  assignedUserId,
+  prisma,
+}: {
+  destinationCalendarObject: DestinationCalendarType;
+  parsedBody: z.infer<typeof schemaDestinationCalendarEditBodyParams>;
+  assignedUserId: number;
+  prisma: PrismaClient;
+}) {
+  if (parsedBody.eventTypeId) {
+    if (!destinationCalendarObject.eventTypeId) {
+      throw new HttpError({
+        statusCode: 400,
+        message: `The provided destination calendar can not be linked to an event type`,
+      });
+    }
+
+    const userEventType = await prisma.eventType.findFirst({
+      where: { id: parsedBody.eventTypeId },
+      select: { userId: true },
+    });
+
+    if (!userEventType || userEventType.userId !== assignedUserId) {
+      throw new HttpError({
+        statusCode: 404,
+        message: `Event type with ID ${parsedBody.eventTypeId} not found`,
+      });
+    }
+  }
+
+  if (!parsedBody.eventTypeId) {
+    if (destinationCalendarObject.eventTypeId) {
+      throw new HttpError({
+        statusCode: 400,
+        message: `The provided destination calendar can only be linked to an event type`,
+      });
+    }
+    if (destinationCalendarObject.userId !== assignedUserId) {
+      throw new HttpError({
+        statusCode: 403,
+        message: `Forbidden`,
+      });
+    }
+  }
+}
+
+/**
+ * Fetches the destination calendar based on the provided ID as the path parameter, specifically `credentialId` and `eventTypeId`.
+ *
+ * If no matching destination calendar is found for the provided ID, an HTTP error with a 404 status
+ * indicating that the desired destination calendar was not found is thrown.
+ *
+ * @param id - The ID of the destination calendar to be retrieved.
+ * @param prisma - An instance of PrismaClient for database operations.
+ *
+ * @returns - An object containing details of the matching destination calendar, specifically `credentialId` and `eventTypeId`.
+ *
+ * @throws HttpError - If no destination calendar matches the provided ID.
+ */
+async function getDestinationCalendar(id: number, prisma: PrismaClient) {
+  const destinationCalendarObject = await prisma.destinationCalendar.findFirst({
+    where: {
+      id,
+    },
+    select: { userId: true, eventTypeId: true, credentialId: true },
+  });
+
+  if (!destinationCalendarObject) {
+    throw new HttpError({
+      statusCode: 404,
+      message: `Destination calendar with ID ${id} not found`,
+    });
+  }
+
+  return destinationCalendarObject;
+}
+
+function validateIntegrationInput(parsedBody: z.infer<typeof schemaDestinationCalendarEditBodyParams>) {
+  if (parsedBody.integration && !parsedBody.externalId) {
+    throw new HttpError({ statusCode: 400, message: "External Id is required with integration value" });
+  }
+  if (!parsedBody.integration && parsedBody.externalId) {
+    throw new HttpError({ statusCode: 400, message: "Integration value is required with external ID" });
+  }
+}
+
 export default defaultResponder(patchHandler);
diff --git a/apps/api/pages/api/destination-calendars/_post.ts b/apps/api/pages/api/destination-calendars/_post.ts
index beccedc30a..40d3cf5e95 100644
--- a/apps/api/pages/api/destination-calendars/_post.ts
+++ b/apps/api/pages/api/destination-calendars/_post.ts
@@ -1,7 +1,9 @@
 import type { NextApiRequest } from "next";
 
+import { getCalendarCredentials, getConnectedCalendars } from "@calcom/core/CalendarManager";
 import { HttpError } from "@calcom/lib/http-error";
 import { defaultResponder } from "@calcom/lib/server";
+import { credentialForCalendarServiceSelect } from "@calcom/prisma/selects/credential";
 
 import {
   schemaDestinationCalendarReadPublic,
@@ -38,9 +40,6 @@ import {
  *               externalId:
  *                 type: string
  *                 description: 'The external ID of the integration'
- *               credentialId:
- *                 type: integer
- *                 description: 'The credential ID it is associated with'
  *               eventTypeId:
  *                 type: integer
  *                 description: 'The ID of the eventType it is associated with'
@@ -65,20 +64,38 @@ async function postHandler(req: NextApiRequest) {
   const parsedBody = schemaDestinationCalendarCreateBodyParams.parse(body);
   await checkPermissions(req, userId);
 
-  const assignedUserId = isAdmin ? parsedBody.userId || userId : userId;
+  const assignedUserId = isAdmin && parsedBody.userId ? parsedBody.userId : userId;
 
   /* Check if credentialId data matches the ownership and integration passed in */
-  const credential = await prisma.credential.findFirst({
-    where: { type: parsedBody.integration, userId: assignedUserId },
-    select: { id: true, type: true, userId: true },
+  const userCredentials = await prisma.credential.findMany({
+    where: {
+      type: parsedBody.integration,
+      userId: assignedUserId,
+    },
+    select: credentialForCalendarServiceSelect,
   });
 
-  if (!credential)
+  if (userCredentials.length === 0)
     throw new HttpError({
       statusCode: 400,
       message: "Bad request, credential id invalid",
     });
 
+  const calendarCredentials = getCalendarCredentials(userCredentials);
+
+  const { connectedCalendars } = await getConnectedCalendars(calendarCredentials, [], parsedBody.externalId);
+
+  const eligibleCalendars = connectedCalendars[0]?.calendars?.filter((calendar) => !calendar.readOnly);
+  const calendar = eligibleCalendars?.find(
+    (c) => c.externalId === parsedBody.externalId && c.integration === parsedBody.integration
+  );
+  if (!calendar?.credentialId)
+    throw new HttpError({
+      statusCode: 400,
+      message: "Bad request, credential id invalid",
+    });
+  const credentialId = calendar.credentialId;
+
   if (parsedBody.eventTypeId) {
     const eventType = await prisma.eventType.findFirst({
       where: { id: parsedBody.eventTypeId, userId: parsedBody.userId },
@@ -91,7 +108,9 @@ async function postHandler(req: NextApiRequest) {
     parsedBody.userId = undefined;
   }
 
-  const destination_calendar = await prisma.destinationCalendar.create({ data: { ...parsedBody } });
+  const destination_calendar = await prisma.destinationCalendar.create({
+    data: { ...parsedBody, credentialId },
+  });
 
   return {
     destinationCalendar: schemaDestinationCalendarReadPublic.parse(destination_calendar),