diff --git a/chrome/content/zotero/xpcom/annotations.js b/chrome/content/zotero/xpcom/annotations.js
index cbe5621e35..e5b0137f85 100644
--- a/chrome/content/zotero/xpcom/annotations.js
+++ b/chrome/content/zotero/xpcom/annotations.js
@@ -32,7 +32,8 @@ Zotero.Annotations = new function () {
 	Zotero.defineProperty(this, 'ANNOTATION_TYPE_NOTE', { value: 2 });
 	Zotero.defineProperty(this, 'ANNOTATION_TYPE_IMAGE', { value: 3 });
 	Zotero.defineProperty(this, 'ANNOTATION_TYPE_INK', { value: 4 });
-	
+	Zotero.defineProperty(this, 'ANNOTATION_TYPE_UNDERLINE', { value: 5 });
+
 	Zotero.defineProperty(this, 'DEFAULT_COLOR', { value: '#ffd400' });
 	
 	Zotero.defineProperty(this, 'PROPS', {
@@ -137,7 +138,7 @@ Zotero.Annotations = new function () {
 			}
 		}
 		o.readOnly = o.isExternal || !isAuthor;
-		if (o.type == 'highlight') {
+		if (['highlight', 'underline'].includes(o.type)) {
 			o.text = item.annotationText;
 		}
 		else if (['image', 'ink'].includes(o.type)) {
@@ -213,7 +214,7 @@ Zotero.Annotations = new function () {
 		item._requireData('annotationDeferred');
 		item.annotationType = json.type;
 		item.annotationAuthorName = json.authorName || '';
-		if (json.type == 'highlight') {
+		if (['highlight', 'underline'].includes(json.type)) {
 			item.annotationText = json.text;
 		}
 		item.annotationIsExternal = !!json.isExternal;
diff --git a/chrome/content/zotero/xpcom/data/item.js b/chrome/content/zotero/xpcom/data/item.js
index de3235d635..8f6deecbbf 100644
--- a/chrome/content/zotero/xpcom/data/item.js
+++ b/chrome/content/zotero/xpcom/data/item.js
@@ -3815,7 +3815,7 @@ for (let name of ['type', 'authorName', 'text', 'comment', 'color', 'pageLabel',
 					if (currentType && currentType != value) {
 						throw new Error("Cannot change annotation type");
 					}
-					if (!['highlight', 'note', 'image', 'ink'].includes(value)) {
+					if (!['highlight', 'underline', 'note', 'image', 'ink'].includes(value)) {
 						let e = new Error(`Unknown annotation type '${value}'`);
 						e.name = "ZoteroInvalidDataError";
 						throw e;
@@ -3823,8 +3823,8 @@ for (let name of ['type', 'authorName', 'text', 'comment', 'color', 'pageLabel',
 					break;
 				}
 				case 'text':
-					if (this._getLatestField('annotationType') != 'highlight') {
-						throw new Error("'annotationText' can only be set for highlight annotations");
+					if (!['highlight', 'underline'].includes(this._getLatestField('annotationType'))) {
+						throw new Error("'annotationText' can only be set for highlight and underline annotations");
 					}
 					break;
 				
@@ -5047,7 +5047,6 @@ Zotero.Item.prototype.fromJSON = function (json, options = {}) {
 		// Annotation fields
 		//
 		case 'annotationType':
-		case 'annotationType':
 		case 'annotationAuthorName':
 		case 'annotationText':
 		case 'annotationComment':
@@ -5326,7 +5325,7 @@ Zotero.Item.prototype.toJSON = function (options = {}) {
 			let type = this.annotationType;
 			obj.annotationType = type;
 			obj.annotationAuthorName = this.annotationAuthorName || '';
-			if (type == 'highlight') {
+			if (['highlight', 'underline'].includes(type)) {
 				obj.annotationText = this.annotationText || '';
 			}
 			obj.annotationComment = this.annotationComment || '';
diff --git a/chrome/content/zotero/xpcom/data/items.js b/chrome/content/zotero/xpcom/data/items.js
index 8fc1e13417..a8513bc2df 100644
--- a/chrome/content/zotero/xpcom/data/items.js
+++ b/chrome/content/zotero/xpcom/data/items.js
@@ -524,6 +524,10 @@ Zotero.Items = function() {
 							case Zotero.Annotations.ANNOTATION_TYPE_HIGHLIGHT:
 								type = 'highlight';
 								break;
+
+							case Zotero.Annotations.ANNOTATION_TYPE_UNDERLINE:
+								type = 'underline';
+								break;
 							
 							case Zotero.Annotations.ANNOTATION_TYPE_NOTE:
 								type = 'note';