diff --git a/AnkiDroid/src/main/java/com/ichi2/anki/dialogs/tags/TagsDialog.kt b/AnkiDroid/src/main/java/com/ichi2/anki/dialogs/tags/TagsDialog.kt
index 1891cdf07e15..0af4163f658f 100644
--- a/AnkiDroid/src/main/java/com/ichi2/anki/dialogs/tags/TagsDialog.kt
+++ b/AnkiDroid/src/main/java/com/ichi2/anki/dialogs/tags/TagsDialog.kt
@@ -22,6 +22,7 @@ import androidx.core.content.ContextCompat
 import androidx.core.os.BundleCompat
 import androidx.core.os.bundleOf
 import androidx.core.view.isVisible
+import androidx.core.widget.doAfterTextChanged
 import androidx.fragment.app.viewModels
 import androidx.lifecycle.flowWithLifecycle
 import androidx.lifecycle.lifecycleScope
@@ -372,6 +373,8 @@ class TagsDialog : AnalyticsDialogFragment {
      * @param prefixTag: The tag to be prefilled into the EditText section. A trailing '::' will be appended.
      */
     @NeedsTest("The prefixTag should be prefilled properly")
+    @NeedsTest("Entering an existing tag should show an error and disable the OK button")
+    @NeedsTest("Entering a new valid tag should clear the error and enable the OK button")
     private fun createAddTagDialog(prefixTag: String?) {
         val addTagDialog =
             AlertDialog
@@ -396,6 +399,46 @@ class TagsDialog : AnalyticsDialogFragment {
             inputET.setText("$prefixTag ")
         }
         inputET.moveCursorToEnd()
+        val positiveButton =
+            addTagDialog.getButton(AlertDialog.BUTTON_POSITIVE)
+
+        positiveButton.isEnabled = false
+
+        val textInputLayout =
+            inputET.parent?.parent
+                as? com.google.android.material.textfield.TextInputLayout
+
+        inputET.doAfterTextChanged { text ->
+
+            val rawTag = text?.toString()?.trim()
+
+            if (rawTag.isNullOrEmpty()) {
+                textInputLayout?.error = null
+                positiveButton.isEnabled = false
+                return@doAfterTextChanged
+            }
+
+            lifecycleScope.launch {
+                val tags = viewModel.tags.await()
+
+                val normalized =
+                    TagsUtil.getUniformedTag(rawTag)
+
+                val exists =
+                    tags.contains(normalized)
+
+                if (exists) {
+                    textInputLayout?.error =
+                        getString(R.string.tag_already_exists)
+
+                    positiveButton.isEnabled = false
+                } else {
+                    textInputLayout?.error = null
+
+                    positiveButton.isEnabled = true
+                }
+            }
+        }
         addTagDialog.show()
     }
 
diff --git a/AnkiDroid/src/main/res/values/02-strings.xml b/AnkiDroid/src/main/res/values/02-strings.xml
index 8c8882821128..5f6a13d1506f 100644
--- a/AnkiDroid/src/main/res/values/02-strings.xml
+++ b/AnkiDroid/src/main/res/values/02-strings.xml
@@ -190,6 +190,7 @@
     <string name="dyn_deck_desc">This is a special deck for studying outside of the normal schedule. Cards will be automatically returned to their original decks after you review them. Deleting this deck from the deck list will return all remaining cards to their original deck.</string>
     <string name="tag_editor_add_feedback">Touch “%2$s” to confirm adding “%1$s”</string>
     <string name="tag_editor_add_feedback_existing">Existing tag “%1$s” selected</string>
+    <string name="tag_already_exists">Tag already exists</string>
     <!-- Time spans -->
 
     <!-- Currently only used if exporting APKG fails -->