diff --git a/doc/whats_new/v1.1.rst b/doc/whats_new/v1.1.rst
index c4f16f4404963..c34c9779e12ec 100644
--- a/doc/whats_new/v1.1.rst
+++ b/doc/whats_new/v1.1.rst
@@ -113,6 +113,10 @@ Changelog
   :pr:`20880` by :user:`Guillaume Lemaitre <glemaitre>`
   and :user:`András Simon <simonandras>`.
 
+- |Enhancement| :func:`utils.estimator_html_repr` shows a more helpful error
+  message when running in a jupyter notebook that is not trusted. :pr:`21316`
+  by `Thomas Fan`_.
+
 Code and Documentation Contributors
 -----------------------------------
 
diff --git a/sklearn/utils/_estimator_html_repr.py b/sklearn/utils/_estimator_html_repr.py
index b2d38b9e97ab3..1f5c339c12771 100644
--- a/sklearn/utils/_estimator_html_repr.py
+++ b/sklearn/utils/_estimator_html_repr.py
@@ -309,6 +309,9 @@ def _write_estimator_html(
   display: inline-block;
   position: relative;
 }
+#$id div.sk-text-repr-fallback {
+  display: none;
+}
 """.replace(
     "  ", ""
 ).replace(
@@ -335,16 +338,33 @@ def estimator_html_repr(estimator):
         container_id = "sk-" + str(uuid.uuid4())
         style_template = Template(_STYLE)
         style_with_id = style_template.substitute(id=container_id)
+        estimator_str = str(estimator)
+
+        # The fallback message is shown by default and loading the CSS sets
+        # div.sk-text-repr-fallback to display: none to hide the fallback message.
+        #
+        # If the notebook is trusted, the CSS is loaded which hides the fallback
+        # message. If the notebook is not trusted, then the CSS is not loaded and the
+        # fallback message is shown by default.
+        #
+        # The reverse logic applies to HTML repr div.sk-container.
+        # div.sk-container is hidden by default and the loading the CSS displays it.
+        fallback_msg = (
+            "Please rerun this cell to show the HTML repr or trust the notebook."
+        )
         out.write(
             f"<style>{style_with_id}</style>"
             f'<div id="{container_id}" class"sk-top-container">'
-            '<div class="sk-container">'
+            '<div class="sk-text-repr-fallback">'
+            f"<pre>{html.escape(estimator_str)}</pre><b>{fallback_msg}</b>"
+            "</div>"
+            '<div class="sk-container" hidden>'
         )
         _write_estimator_html(
             out,
             estimator,
             estimator.__class__.__name__,
-            str(estimator),
+            estimator_str,
             first_call=True,
         )
         out.write("</div></div>")
diff --git a/sklearn/utils/tests/test_estimator_html_repr.py b/sklearn/utils/tests/test_estimator_html_repr.py
index f22c03f20bdd7..9d474ad10fe10 100644
--- a/sklearn/utils/tests/test_estimator_html_repr.py
+++ b/sklearn/utils/tests/test_estimator_html_repr.py
@@ -1,4 +1,5 @@
 from contextlib import closing
+import html
 from io import StringIO
 
 import pytest
@@ -278,3 +279,14 @@ def test_one_estimator_print_change_only(print_changed_only):
         pca_repr = str(pca)
         html_output = estimator_html_repr(pca)
         assert pca_repr in html_output
+
+
+def test_fallback_exists():
+    """Check that repr fallback is in the HTML."""
+    pca = PCA(n_components=10)
+    html_output = estimator_html_repr(pca)
+
+    assert (
+        f'<div class="sk-text-repr-fallback"><pre>{html.escape(str(pca))}'
+        in html_output
+    )