diff --git a/src/ajax.js b/src/ajax.js
index 639a6a3807..7e8f552aac 100644
--- a/src/ajax.js
+++ b/src/ajax.js
@@ -855,5 +855,14 @@ jQuery.each( [ "get", "post" ], function( _i, method ) {
 	};
 } );
 
+jQuery.ajaxPrefilter( function( s ) {
+	var i;
+	for ( i in s.headers ) {
+		if ( i.toLowerCase() === "content-type" ) {
+			s.contentType = s.headers[ i ] || "";
+		}
+	}
+} );
+
 return jQuery;
 } );
diff --git a/test/unit/ajax.js b/test/unit/ajax.js
index b91ef6711b..d9b984799c 100644
--- a/test/unit/ajax.js
+++ b/test/unit/ajax.js
@@ -1326,6 +1326,52 @@ QUnit.module( "ajax", {
 		};
 	} );
 
+	ajaxTest( "jQuery.ajax() - don't escape %20 with contentType override (gh-4119)", 1, function( assert ) {
+		return {
+			url: "bogus.html",
+			contentType: "application/x-www-form-urlencoded",
+			headers: { "content-type": "application/json" },
+			method: "post",
+			dataType: "json",
+			data: "{\"val\":\"%20\"}",
+			beforeSend: function( _, s ) {
+				assert.strictEqual( s.data, "{\"val\":\"%20\"}", "data is not %20-encoded" );
+				return false;
+			},
+			error: true
+		};
+	} );
+
+	ajaxTest( "jQuery.ajax() - escape %20 with contentType override (gh-4119)", 1, function( assert ) {
+		return {
+			url: "bogus.html",
+			contentType: "application/json",
+			headers: { "content-type": "application/x-www-form-urlencoded" },
+			method: "post",
+			dataType: "json",
+			data: "{\"val\":\"%20\"}",
+			beforeSend: function( _, s ) {
+				assert.strictEqual( s.data, "{\"val\":\"+\"}", "data is %20-encoded" );
+				return false;
+			},
+			error: true
+		};
+	} );
+
+	ajaxTest( "jQuery.ajax() - override contentType with header (gh-4119)", 1, function( assert ) {
+		return {
+			url: "bogus.html",
+			contentType: "application/json",
+			headers: { "content-type": "application/x-www-form-urlencoded" },
+			beforeSend: function( _, s ) {
+				assert.strictEqual( s.contentType, "application/x-www-form-urlencoded",
+					"contentType is overwritten" );
+				return false;
+			},
+			error: true
+		};
+	} );
+
 	ajaxTest( "jQuery.ajax() - data - no processing POST", 1, function( assert ) {
 		return {
 			url: "bogus.html",