diff --git a/src/Api/Vault/Controllers/CiphersController.cs b/src/Api/Vault/Controllers/CiphersController.cs
index c0a974bce2..06c88ad9bb 100644
--- a/src/Api/Vault/Controllers/CiphersController.cs
+++ b/src/Api/Vault/Controllers/CiphersController.cs
@@ -887,6 +887,9 @@ public class CiphersController : Controller
     [HttpPost("bulk-collections")]
     public async Task PostBulkCollections([FromBody] CipherBulkUpdateCollectionsRequestModel model)
     {
+        var userId = _userService.GetProperUserId(User).Value;
+        await _cipherService.ValidateBulkCollectionAssignmentAsync(model.CollectionIds, model.CipherIds, userId);
+
         if (!await CanModifyCipherCollectionsAsync(model.OrganizationId, model.CipherIds) ||
             !await CanEditItemsInCollections(model.OrganizationId, model.CollectionIds))
         {
diff --git a/src/Core/Vault/Services/ICipherService.cs b/src/Core/Vault/Services/ICipherService.cs
index dac535433c..ffd79e9381 100644
--- a/src/Core/Vault/Services/ICipherService.cs
+++ b/src/Core/Vault/Services/ICipherService.cs
@@ -37,4 +37,5 @@ public interface ICipherService
     Task UploadFileForExistingAttachmentAsync(Stream stream, Cipher cipher, CipherAttachment.MetaData attachmentId);
     Task<AttachmentResponseData> GetAttachmentDownloadDataAsync(Cipher cipher, string attachmentId);
     Task<bool> ValidateCipherAttachmentFile(Cipher cipher, CipherAttachment.MetaData attachmentData);
+    Task ValidateBulkCollectionAssignmentAsync(IEnumerable<Guid> collectionIds, IEnumerable<Guid> cipherIds, Guid userId);
 }
diff --git a/src/Core/Vault/Services/Implementations/CipherService.cs b/src/Core/Vault/Services/Implementations/CipherService.cs
index ebfb2a4a2a..35b745fce6 100644
--- a/src/Core/Vault/Services/Implementations/CipherService.cs
+++ b/src/Core/Vault/Services/Implementations/CipherService.cs
@@ -170,6 +170,7 @@ public class CipherService : ICipherService
         {
             ValidateCipherLastKnownRevisionDateAsync(cipher, lastKnownRevisionDate);
             cipher.RevisionDate = DateTime.UtcNow;
+            await ValidateChangeInCollectionsAsync(cipher, collectionIds, savingUserId);
             await ValidateViewPasswordUserAsync(cipher);
             await _cipherRepository.ReplaceAsync(cipher);
             await _eventService.LogCipherEventAsync(cipher, Bit.Core.Enums.EventType.Cipher_Updated);
@@ -539,6 +540,7 @@ public class CipherService : ICipherService
         try
         {
             await ValidateCipherCanBeShared(cipher, sharingUserId, organizationId, lastKnownRevisionDate);
+            await ValidateChangeInCollectionsAsync(cipher, collectionIds, sharingUserId);
 
             // Sproc will not save this UserId on the cipher. It is used limit scope of the collectionIds.
             cipher.UserId = sharingUserId;
@@ -678,6 +680,7 @@ public class CipherService : ICipherService
         {
             throw new BadRequestException("Cipher must belong to an organization.");
         }
+        await ValidateChangeInCollectionsAsync(cipher, collectionIds, savingUserId);
 
         cipher.RevisionDate = DateTime.UtcNow;
 
@@ -820,6 +823,15 @@ public class CipherService : ICipherService
         return restoringCiphers;
     }
 
+    public async Task ValidateBulkCollectionAssignmentAsync(IEnumerable<Guid> collectionIds, IEnumerable<Guid> cipherIds, Guid userId)
+    {
+        foreach (var cipherId in cipherIds)
+        {
+            var cipher = await _cipherRepository.GetByIdAsync(cipherId);
+            await ValidateChangeInCollectionsAsync(cipher, collectionIds, userId);
+        }
+    }
+
     private async Task<bool> UserCanEditAsync(Cipher cipher, Guid userId)
     {
         if (!cipher.OrganizationId.HasValue && cipher.UserId.HasValue && cipher.UserId.Value == userId)
@@ -1038,6 +1050,44 @@ public class CipherService : ICipherService
         }
     }
 
+    // Validates that a cipher is not being added to a default collection when it is only currently only in shared collections
+    private async Task ValidateChangeInCollectionsAsync(Cipher updatedCipher, IEnumerable<Guid> newCollectionIds, Guid userId)
+    {
+
+        if (updatedCipher.Id == Guid.Empty || !updatedCipher.OrganizationId.HasValue)
+        {
+            return;
+        }
+
+        var currentCollectionsForCipher = await _collectionCipherRepository.GetManyByUserIdCipherIdAsync(userId, updatedCipher.Id);
+
+        if (!currentCollectionsForCipher.Any())
+        {
+            // When a cipher is not currently in any collections it can be assigned to any type of collection
+            return;
+        }
+
+        var currentCollections = await _collectionRepository.GetManyByManyIdsAsync(currentCollectionsForCipher.Select(c => c.CollectionId));
+
+        var currentCollectionsContainDefault = currentCollections.Any(c => c.Type == CollectionType.DefaultUserCollection);
+
+        // When the current cipher already contains the default collection, no check is needed for if they added or removed
+        // a default collection, because it is already there.
+        if (currentCollectionsContainDefault)
+        {
+            return;
+        }
+
+        var newCollections = await _collectionRepository.GetManyByManyIdsAsync(newCollectionIds);
+        var newCollectionsContainDefault = newCollections.Any(c => c.Type == CollectionType.DefaultUserCollection);
+
+        if (newCollectionsContainDefault)
+        {
+            // User is trying to add the default collection when the cipher is only in shared collections
+            throw new BadRequestException("The cipher(s) cannot be assigned to a default collection when only assigned to non-default collections.");
+        }
+    }
+
     private string SerializeCipherData(CipherData data)
     {
         return data switch