diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9503651a32f..92e95165c42 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -24,6 +24,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   - Fix issue with per-point marker color for hover labels in `scattergl`, `quiver` traces [[#8027](https://github.com/plotly/plotly.js/pull/8027)]
   - Update `maplibre-gl` to v6 to address [CVE-2026-85061](https://github.com/advisories/GHSA-jrc7-96c5-q579) [[#8035](https://github.com/plotly/plotly.js/pull/8035)]
     - Note: Safari 15, Chrome 56, Firefox 51 and later are now required for map traces
+- Update `hex_to_rgb` function to raise error for invalid-length hex codes, and emit warning for hex codes containing alpha [[#5729](https://github.com/plotly/plotly.py/pull/5729)], with thanks to @dylanpulver for the contribution!
 
 ## [7.0.0] - 2026-08-25
 
diff --git a/_plotly_utils/colors/__init__.py b/_plotly_utils/colors/__init__.py
index e3b5cfa3205..a9689101e6b 100644
--- a/_plotly_utils/colors/__init__.py
+++ b/_plotly_utils/colors/__init__.py
@@ -76,6 +76,7 @@
 
 import decimal
 from numbers import Number
+from warnings import warn
 
 from _plotly_utils import exceptions
 
@@ -763,15 +764,29 @@ def hex_to_rgb(value):
         '#FFF'    --> (255, 255, 255)
 
     """
+
+    input_value = value
     value = value.lstrip("#")
     if len(value) == 3:
         value = "".join(c * 2 for c in value)
-    hex_total_length = len(value)
-    rgb_section_length = hex_total_length // 3
-    return tuple(
-        int(value[i : i + rgb_section_length], 16)
-        for i in range(0, hex_total_length, rgb_section_length)
-    )
+    elif len(value) == 4:
+        warn(
+            "4-character hex color provided; 4th character will be ignored."
+            "got {!r}".format(input_value)
+        )
+        value = "".join(c * 2 for c in value)[:6]
+    elif len(value) == 8:
+        warn(
+            "8-character hex color provided; last two characters will be ignored."
+            "got {!r}".format(input_value)
+        )
+        value = value[:6]
+    elif len(value) != 6:
+        raise ValueError(
+            "hex color must be 3 or 6 hex digits, optionally prefixed with "
+            "'#'; got {!r}".format(input_value)
+        )
+    return tuple(int(value[i : i + 2], 16) for i in range(0, 6, 2))
 
 
 def colorscale_to_colors(colorscale):
diff --git a/tests/test_plotly_utils/colors/test_color_conversions.py b/tests/test_plotly_utils/colors/test_color_conversions.py
index 6513b7c3233..8e28734b045 100644
--- a/tests/test_plotly_utils/colors/test_color_conversions.py
+++ b/tests/test_plotly_utils/colors/test_color_conversions.py
@@ -1,3 +1,5 @@
+import pytest
+
 from _plotly_utils.colors import (
     find_intermediate_color,
     hex_to_rgb,
@@ -21,6 +23,26 @@ def test_hex_to_rgb_shorthand_3_digit():
     assert hex_to_rgb("#00f") == (0, 0, 255)
 
 
+@pytest.mark.parametrize("value", ["#abcd", "eb4d", "#12345678", "b24fa3d1"])
+def test_hex_to_rgb_warns_on_4_and_8_digits(value):
+    warning_must_contain = "4-char" if len(value.lstrip("#")) == 4 else "8-char"
+    with pytest.warns(UserWarning, match=warning_must_contain):
+        hex_to_rgb(value)
+
+
+@pytest.mark.parametrize("value", ["#12345", "#1", "#1234567", "", "#"])
+def test_hex_to_rgb_rejects_other_lengths(value):
+    # The section width was len // 3, so "#12345" returned a 5-tuple rather
+    # than raising.
+    with pytest.raises(ValueError, match="3 or 6 hex digits"):
+        hex_to_rgb(value)
+
+
+def test_hex_to_rgb_accepts_missing_hash():
+    assert hex_to_rgb("aabbcc") == (170, 187, 204)
+    assert hex_to_rgb("abc") == (170, 187, 204)
+
+
 def test_label_rgb_formats_tuple():
     assert label_rgb((255, 0, 0)) == "rgb(255, 0, 0)"
     assert label_rgb((1, 2, 3)) == "rgb(1, 2, 3)"