diff --git a/coderd/database/dbmem/dbmem.go b/coderd/database/dbmem/dbmem.go
index f83838dd4b..04eecd5d86 100644
--- a/coderd/database/dbmem/dbmem.go
+++ b/coderd/database/dbmem/dbmem.go
@@ -6197,6 +6197,20 @@ func (q *FakeQuerier) InsertOrganizationMember(_ context.Context, arg database.I
 	q.mutex.Lock()
 	defer q.mutex.Unlock()
 
+	if slices.IndexFunc(q.data.organizationMembers, func(member database.OrganizationMember) bool {
+		return member.OrganizationID == arg.OrganizationID && member.UserID == arg.UserID
+	}) >= 0 {
+		// Error pulled from a live db error
+		return database.OrganizationMember{}, &pq.Error{
+			Severity:   "ERROR",
+			Code:       "23505",
+			Message:    "duplicate key value violates unique constraint \"organization_members_pkey\"",
+			Detail:     "Key (organization_id, user_id)=(f7de1f4e-5833-4410-a28d-0a105f96003f, 36052a80-4a7f-4998-a7ca-44cefa608d3e) already exists.",
+			Table:      "organization_members",
+			Constraint: "organization_members_pkey",
+		}
+	}
+
 	//nolint:gosimple
 	organizationMember := database.OrganizationMember{
 		OrganizationID: arg.OrganizationID,
diff --git a/coderd/members.go b/coderd/members.go
index 3352eefc3b..2528a17878 100644
--- a/coderd/members.go
+++ b/coderd/members.go
@@ -43,6 +43,12 @@ func (api *API) postOrganizationMember(rw http.ResponseWriter, r *http.Request)
 		httpapi.ResourceNotFound(rw)
 		return
 	}
+	if database.IsUniqueViolation(err, database.UniqueOrganizationMembersPkey) {
+		httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
+			Message: "Organization member already exists in this organization",
+		})
+		return
+	}
 	if err != nil {
 		httpapi.InternalServerError(rw, err)
 		return
diff --git a/coderd/members_test.go b/coderd/members_test.go
index db80b28ad1..3db296ef60 100644
--- a/coderd/members_test.go
+++ b/coderd/members_test.go
@@ -50,6 +50,19 @@ func TestAddMember(t *testing.T) {
 			db2sdk.List(members, onlyIDs))
 	})
 
+	t.Run("AlreadyMember", func(t *testing.T) {
+		t.Parallel()
+		owner := coderdtest.New(t, nil)
+		first := coderdtest.CreateFirstUser(t, owner)
+		_, user := coderdtest.CreateAnotherUser(t, owner, first.OrganizationID)
+
+		ctx := testutil.Context(t, testutil.WaitMedium)
+		// Add user to org, even though they already exist
+		// nolint:gocritic // must be an owner to see the user
+		_, err := owner.PostOrganizationMember(ctx, first.OrganizationID, user.Username)
+		require.ErrorContains(t, err, "already exists")
+	})
+
 	t.Run("UserNotExists", func(t *testing.T) {
 		t.Parallel()
 		owner := coderdtest.New(t, nil)