diff --git a/src/replacements/gcn-circular/index.ts b/src/replacements/gcn-circular/index.ts
index dc0d013..222d16e 100644
--- a/src/replacements/gcn-circular/index.ts
+++ b/src/replacements/gcn-circular/index.ts
@@ -9,13 +9,22 @@ const preamble = /GCN(?:s|\s+Circulars?|\s+Circs?\.)?\s*/.source
 const conjunction = /(?:\s*|,|and)+/.source
 const circularIdRegExp = /\d+(?:a|\.5)?/gi
 const circularId = circularIdRegExp.source
+const legacyUrlOrigin = /https?:\/\/gcn\.gsfc\.nasa\.gov/.source
+const legacyUrlExtension = /\.gcn3/.source
+const legacyUrl = `${legacyUrlOrigin}/gcn3/(${circularId})${legacyUrlExtension}`
+const urlOrigin = /https?:\/\/gcn.nasa.gov/.source
+const url = `${urlOrigin}/circulars/(${circularId})`
 
 export default {
   find: new RegExp(
-    `${preamble}${circularId}(?:${conjunction}${circularId})*`,
+    `${url}|${legacyUrl}|${preamble}${circularId}(?:${conjunction}${circularId})*`,
     'gi'
   ),
-  replace(data, value) {
+  replace(data, value, circularIdFromUrl, circularIdFromLegacyUrl) {
+    circularIdFromUrl ||= circularIdFromLegacyUrl
+    if (circularIdFromUrl)
+      return data('gcn-circular', value, normalize(circularIdFromUrl))
+
     const result = []
     let lastIndex = 0
     for (const match of value.matchAll(circularIdRegExp)) {
diff --git a/src/replacements/gcn-circular/test.html b/src/replacements/gcn-circular/test.html
index 45b24fb..990277c 100644
--- a/src/replacements/gcn-circular/test.html
+++ b/src/replacements/gcn-circular/test.html
@@ -1,4 +1,6 @@
 <h1>Hello, world</h1>
 <p>This paragraph refers to GCN Circular <data class="gcn-circular" value="123">123</data> and GCN <data class="gcn-circular" value="456">456</data></p>
+<p>You can refer to a Circular by its URL: <data class="gcn-circular" value="26640">https://gcn.nasa.gov/circulars/26640</data></p>
+<p>A legacy URL works too: <data class="gcn-circular" value="26640">https://gcn.gsfc.nasa.gov/gcn3/26640.gcn3</data></p>
 <p>GCN Circ. <data class="gcn-circular" value="789">789</data> and GCN Circs. <data class="gcn-circular" value="3">3</data> and <data class="gcn-circular" value="4">4</data></p>
 <p>And this one refers to GCNs <data class="gcn-circular" value="1">1</data>, <data class="gcn-circular" value="23.5">23a</data>, <data class="gcn-circular" value="45.5">45.5</data> and <data class="gcn-circular" value="42">42</data></p>
\ No newline at end of file
diff --git a/src/replacements/gcn-circular/test.json b/src/replacements/gcn-circular/test.json
index 964e7ef..72b549e 100644
--- a/src/replacements/gcn-circular/test.json
+++ b/src/replacements/gcn-circular/test.json
@@ -48,6 +48,40 @@
         }
       ]
     },
+    {
+      "type": "paragraph",
+      "children": [
+        {
+          "type": "text",
+          "value": "You can refer to a Circular by its URL: "
+        },
+        {
+          "type": "text",
+          "value": "https://gcn.nasa.gov/circulars/26640",
+          "data": {
+            "class": "gcn-circular",
+            "value": 26640
+          }
+        }
+      ]
+    },
+    {
+      "type": "paragraph",
+      "children": [
+        {
+          "type": "text",
+          "value": "A legacy URL works too: "
+        },
+        {
+          "type": "text",
+          "value": "https://gcn.gsfc.nasa.gov/gcn3/26640.gcn3",
+          "data": {
+            "class": "gcn-circular",
+            "value": 26640
+          }
+        }
+      ]
+    },
     {
       "type": "paragraph",
       "children": [
diff --git a/src/replacements/gcn-circular/test.md b/src/replacements/gcn-circular/test.md
index 8646b70..feeb040 100644
--- a/src/replacements/gcn-circular/test.md
+++ b/src/replacements/gcn-circular/test.md
@@ -2,6 +2,10 @@
 
 This paragraph refers to GCN Circular 123 and GCN 456
 
+You can refer to a Circular by its URL: https://gcn.nasa.gov/circulars/26640
+
+A legacy URL works too: https://gcn.gsfc.nasa.gov/gcn3/26640.gcn3
+
 GCN Circ. 789 and GCN Circs. 3 and 4
 
 And this one refers to GCNs 1, 23a, 45.5 and 42