diff --git a/CHANGES b/CHANGES
index 1bb4842593..eda0d2bc46 100644
--- a/CHANGES
+++ b/CHANGES
@@ -17,6 +17,10 @@ jsoup changelog
     element, e.g. ancestor-or-self::*.
     <https://github.com/jhy/jsoup/issues/1652>
 
+  * Bugfix: boolean attribute names should be case-insensitive, but were not when the parser was configured to preserve
+    case.
+    <https://github.com/jhy/jsoup/issues/1656>
+
 *** Release 1.14.3 [2021-Sep-30]
   * Improvement: added native XPath support in Element#selectXpath(String)
     <https://github.com/jhy/jsoup/pull/1629>
diff --git a/src/main/java/org/jsoup/nodes/Attribute.java b/src/main/java/org/jsoup/nodes/Attribute.java
index a1b2ae21ec..5cdecc339c 100644
--- a/src/main/java/org/jsoup/nodes/Attribute.java
+++ b/src/main/java/org/jsoup/nodes/Attribute.java
@@ -2,6 +2,7 @@
 
 import org.jsoup.SerializationException;
 import org.jsoup.helper.Validate;
+import org.jsoup.internal.Normalizer;
 import org.jsoup.internal.StringUtil;
 import org.jsoup.nodes.Document.OutputSettings.Syntax;
 
@@ -211,7 +212,7 @@ protected static boolean shouldCollapseAttribute(final String key, @Nullable fin
      * Checks if this attribute name is defined as a boolean attribute in HTML5
      */
     public static boolean isBooleanAttribute(final String key) {
-        return Arrays.binarySearch(booleanAttributes, key) >= 0;
+        return Arrays.binarySearch(booleanAttributes, Normalizer.lowerCase(key)) >= 0;
     }
 
     @Override
diff --git a/src/test/java/org/jsoup/nodes/AttributeTest.java b/src/test/java/org/jsoup/nodes/AttributeTest.java
index 629ed8bf64..454b3bb041 100644
--- a/src/test/java/org/jsoup/nodes/AttributeTest.java
+++ b/src/test/java/org/jsoup/nodes/AttributeTest.java
@@ -1,6 +1,8 @@
 package org.jsoup.nodes;
 
 import org.jsoup.Jsoup;
+import org.jsoup.parser.ParseSettings;
+import org.jsoup.parser.Parser;
 import org.junit.jupiter.api.Test;
 
 import static org.junit.jupiter.api.Assertions.*;
@@ -72,4 +74,19 @@ public void html() {
         oldVal = attr.setValue("foo");
         assertEquals("", oldVal); // string, not null
     }
+
+    @Test void booleanAttributesAreNotCaseSensitive() {
+        // https://github.com/jhy/jsoup/issues/1656
+        assertTrue(Attribute.isBooleanAttribute("required"));
+        assertTrue(Attribute.isBooleanAttribute("REQUIRED"));
+        assertTrue(Attribute.isBooleanAttribute("rEQUIREd"));
+        assertFalse(Attribute.isBooleanAttribute("random string"));
+
+        String html = "<a href=autofocus REQUIRED>One</a>";
+        Document doc = Jsoup.parse(html);
+        assertEquals("<a href=\"autofocus\" required>One</a>", doc.selectFirst("a").outerHtml());
+
+        Document doc2 = Jsoup.parse(html, Parser.htmlParser().settings(ParseSettings.preserveCase));
+        assertEquals("<a href=\"autofocus\" REQUIRED>One</a>", doc2.selectFirst("a").outerHtml());
+    }
 }