diff --git a/CHANGES b/CHANGES
index 74f3fb8b55..8bf13ed6d2 100644
--- a/CHANGES
+++ b/CHANGES
@@ -136,6 +136,7 @@ Bugs fixed
 * #2485: autosummary crashes with multiple source_suffix values
 * #1734: Could not translate the caption of toctree directive
 * Could not translate the content of meta directive (ref: #1734)
+* #2550: external links are opened in help viewer
 
 Documentation
 -------------
diff --git a/sphinx/builders/htmlhelp.py b/sphinx/builders/htmlhelp.py
index cdb51b2c60..ecc752b606 100644
--- a/sphinx/builders/htmlhelp.py
+++ b/sphinx/builders/htmlhelp.py
@@ -198,6 +198,14 @@ class HTMLHelpBuilder(StandaloneHTMLBuilder):
     def handle_finish(self):
         self.build_hhx(self.outdir, self.config.htmlhelp_basename)
 
+    def write_doc(self, docname, doctree):
+        for node in doctree.traverse(nodes.reference):
+            # add ``target=_blank`` attributes to external links
+            if node.get('internal') is None and 'refuri' in node:
+                node['target'] = '_blank'
+
+        StandaloneHTMLBuilder.write_doc(self, docname, doctree)
+
     def build_hhx(self, outdir, outname):
         self.info('dumping stopword list...')
         with self.open_file(outdir, outname+'.stp') as f:
diff --git a/sphinx/writers/html.py b/sphinx/writers/html.py
index 324f8132a2..e15a18ab73 100644
--- a/sphinx/writers/html.py
+++ b/sphinx/writers/html.py
@@ -213,6 +213,8 @@ class HTMLTranslator(BaseTranslator):
             atts['class'] += ' image-reference'
         if 'reftitle' in node:
             atts['title'] = node['reftitle']
+        if 'target' in node:
+            atts['target'] = node['target']
         self.body.append(self.starttag(node, 'a', '', **atts))
 
         if node.get('secnumber'):