diff --git a/core/src/main/java/hudson/Functions.java b/core/src/main/java/hudson/Functions.java
index 720312c88c..cefe42a14c 100644
--- a/core/src/main/java/hudson/Functions.java
+++ b/core/src/main/java/hudson/Functions.java
@@ -2336,6 +2336,10 @@ public class Functions {
         }
 
         if (iconMetadata == null) {
+            if (!iconGuess.startsWith("/") && !iconGuess.startsWith("http")) {
+                iconGuess = "/" + iconGuess;
+            }
+
             iconSource = rootURL + (iconGuess.startsWith("/images/") || iconGuess.startsWith("/plugin/") ? getResourcePath() : "") + iconGuess;
         }
 
diff --git a/test/src/test/java/lib/layout/IconTest.java b/test/src/test/java/lib/layout/IconTest.java
index b54ffffd80..478cf76da4 100644
--- a/test/src/test/java/lib/layout/IconTest.java
+++ b/test/src/test/java/lib/layout/IconTest.java
@@ -151,7 +151,7 @@ public class IconTest  {
         assertIconToSymbolOkay(taskDivs.get(5).getElementsByTagName("svg").get(0));
 
         assertIconToImageOkay(taskDivs.get(6).getElementsByTagName("img").get(0), "/plugin/xxx/icon.png");
-        assertIconToImageOkay(taskDivs.get(7).getElementsByTagName("img").get(0), "plugin/xxx/icon.png");
+        assertIconToImageOkay(taskDivs.get(7).getElementsByTagName("img").get(0), "/plugin/xxx/icon.png");
     }
 
     @TestExtension("testTasks")