diff --git a/src/language-css/printer-postcss.js b/src/language-css/printer-postcss.js
index 6f848224..d2e2ef81 100644
--- a/src/language-css/printer-postcss.js
+++ b/src/language-css/printer-postcss.js
@@ -959,6 +959,7 @@ function maybeToLowerCase(value) {
     value.includes("#") ||
     value.startsWith("%") ||
     value.startsWith("--") ||
+    value.startsWith(":--") ||
     (value.includes("(") && value.includes(")"))
     ? value
     : value.toLowerCase();
diff --git a/tests/css_case/__snapshots__/jsfmt.spec.js.snap b/tests/css_case/__snapshots__/jsfmt.spec.js.snap
index aa91143e..d066aac9 100644
--- a/tests/css_case/__snapshots__/jsfmt.spec.js.snap
+++ b/tests/css_case/__snapshots__/jsfmt.spec.js.snap
@@ -748,3 +748,18 @@ table {
 }
 
 `;
+
+exports[`custom-selectors.css 1`] = `
+@custom-selector :--camelCase .my-css-selector;
+
+:--camelCase {
+  content: red;
+}
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+@custom-selector :--camelCase .my-css-selector;
+
+:--camelCase {
+  content: red;
+}
+
+`;
diff --git a/tests/css_case/custom-selectors.css b/tests/css_case/custom-selectors.css
new file mode 100644
index 00000000..30176f54
--- /dev/null
+++ b/tests/css_case/custom-selectors.css
@@ -0,0 +1,5 @@
+@custom-selector :--camelCase .my-css-selector;
+
+:--camelCase {
+  content: red;
+}