diff --git a/chrome/content/zotero/runJS.html b/chrome/content/zotero/runJS.html
index 509e279c08..86d55bfc38 100644
--- a/chrome/content/zotero/runJS.html
+++ b/chrome/content/zotero/runJS.html
@@ -28,8 +28,9 @@
 			</div>
 			
 			<div class="textbox-container">
-				<div class="textbox-header">
-					<label id="result-label" class="textbox-label" for="result" data-l10n-id="runJS-result"/>
+				<div class="textbox-header result-header">
+					<label id="result-label" class="textbox-label" for="result" data-l10n-id="runJS-result"></label>
+					<div id="loading-spinner" class="zotero-spinner-16"></div>
 				</div>
 				<textarea id="result" readonly aria-disabled="true"></textarea>
 			</div>
diff --git a/chrome/content/zotero/runJS.js b/chrome/content/zotero/runJS.js
index dfe8a8ccb9..c14b391092 100644
--- a/chrome/content/zotero/runJS.js
+++ b/chrome/content/zotero/runJS.js
@@ -14,6 +14,8 @@ async function run() {
 	var isAsync = document.getElementById('run-as-async').checked;
 	var result;
 	var resultTextbox = document.getElementById('result');
+	var spinner = document.getElementById("loading-spinner");
+	spinner.setAttribute("status", "animate");
 	try {
 		if (isAsync) {
 			code = '(async function () {' + code + '})()';
@@ -26,10 +28,27 @@ async function run() {
 	catch (e) {
 		resultTextbox.classList.add('error');
 		resultTextbox.textContent = e;
+		spinner.removeAttribute("status");
 		return;
 	}
+	// Hide the spinner after a small delay so it briefly appears even
+	// if the code runs fast to indicate that everything did run
+	setTimeout(() => {
+		spinner.removeAttribute("status");
+	}, 100);
+	
 	resultTextbox.classList.remove('error');
-	resultTextbox.textContent = typeof result == 'string' ? result : Zotero.Utilities.varDump(result);
+	if (typeof result == 'string') {
+		resultTextbox.textContent = result;
+	}
+	else if (result !== undefined) {
+		resultTextbox.textContent = Zotero.Utilities.varDump(result);
+	}
+	else {
+		// when nothing is returned, log undefined as the return value but
+		// for clarity also add a note that the JS run was successful
+		resultTextbox.textContent = `===>undefined<=== (${Zotero.getString("runJS-completed")})`;
+	}
 }
 
 // eslint-disable-next-line no-unused-vars
diff --git a/chrome/locale/en-US/zotero/zotero.ftl b/chrome/locale/en-US/zotero/zotero.ftl
index e17ea34e46..b2430f460e 100644
--- a/chrome/locale/en-US/zotero/zotero.ftl
+++ b/chrome/locale/en-US/zotero/zotero.ftl
@@ -294,6 +294,7 @@ runJS-title = Run JavaScript
 runJS-editor-label = Code:
 runJS-run = Run
 runJS-help = { general-help }
+runJS-completed = completed successfully
 runJS-result = {
     $type ->
         [async] Return value:
diff --git a/scss/components/_runJS.scss b/scss/components/_runJS.scss
index 426f1acf0e..137a97c7da 100644
--- a/scss/components/_runJS.scss
+++ b/scss/components/_runJS.scss
@@ -44,6 +44,14 @@
         line-height: 1.5;
     }
 
+	.result-header {
+		justify-content: start;
+	}
+
+	#loading-spinner {
+		margin-inline-start: 5px;
+	}
+
     .textbox-label {
         font-size: 15px;
     }