diff --git a/guix/swh.scm b/guix/swh.scm
index 4e71bdb045..60e97c6d38 100644
--- a/guix/swh.scm
+++ b/guix/swh.scm
@@ -78,6 +78,14 @@ (define-module (guix swh)
             lookup-revision
             lookup-origin-revision
 
+            external-id?
+            external-id-value
+            external-id-type
+            external-id-version
+            external-id-target
+            lookup-external-id
+            lookup-directory-by-nar-hash
+
             content?
             content-checksums
             content-data-url
@@ -382,6 +390,15 @@ (define-json-mapping <directory-entry> make-directory-entry directory-entry?
   (permissions   directory-entry-permissions "perms")
   (target-url    directory-entry-target-url "target_url"))
 
+;; <https://archive.softwareheritage.org/api/1/extid/doc/>
+(define-json-mapping <external-id> make-external-id external-id?
+  json->external-id
+  (value         external-id-value "extid")
+  (type          external-id-type "extid_type")
+  (version       external-id-version "extid_version")
+  (target        external-id-target)
+  (target-url    external-id-target-url "target_url"))
+
 ;; <https://archive.softwareheritage.org/api/1/origin/save/>
 (define-json-mapping <save-reply> make-save-reply save-reply?
   json->save-reply
@@ -436,6 +453,24 @@ (define (json->directory-entries port)
   (map json->directory-entry
        (vector->list (json->scm port))))
 
+(define (lookup-external-id type id)
+  "Return the external ID record for ID, a bytevector, of the given TYPE
+(currently one of: \"bzr-nodeid\", \"hg-nodeid\", \"nar-sha256\",
+\"checksum-sha512\")."
+  (call (swh-url "/api/1/extid" type
+                 (string-append "hex:" (bytevector->base16-string id)))
+        json->external-id))
+
+(define* (lookup-directory-by-nar-hash hash #:optional (algorithm 'sha256))
+  "Return the SWHID of a directory---i.e., prefixed by \"swh:1:dir\"---for the
+directory that with the given HASH (a bytevector), assuming nar serialization
+and use of ALGORITHM."
+  ;; example:
+  ;; https://archive.softwareheritage.org/api/1/extid/nar-sha256/base64url:0jD6Z4TLMm5g1CviuNNuVNP31KWyoT_oevfr8TQwc3Y/
+  (and=> (lookup-external-id (string-append "nar-" (symbol->string algorithm))
+                             hash)
+         external-id-target))
+
 (define (origin-visits origin)
   "Return the list of visits of ORIGIN, a record as returned by
 'lookup-origin'."
diff --git a/tests/swh.scm b/tests/swh.scm
index a36f951241..e7ced6b50c 100644
--- a/tests/swh.scm
+++ b/tests/swh.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2019-2021, 2024 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -18,6 +18,7 @@
 
 (define-module (test-swh)
   #:use-module (guix swh)
+  #:use-module (guix base32)
   #:use-module (guix tests http)
   #:use-module (web response)
   #:use-module (srfi srfi-19)
@@ -56,6 +57,16 @@ (define %directory-entries
        \"length\": 456,
        \"dir_id\": 2 } ]")
 
+(define %external-id
+  "{ \"extid_type\": \"nar-sha256\",
+     \"extid\":
+\"0b56ba94c2b83b8f74e3772887c1109135802eb3e8962b628377987fe97e1e63\",
+     \"version\": 0,
+     \"target\": \"swh:1:dir:84a8b34591712c0a90bab0af604188bcd1fe3153\",
+     \"target_url\":
+\"https://archive.softwareheritage.org/swh:1:dir:84a8b34591712c0a90bab0af604188bcd1fe3153\"
+   }")
+
 (define-syntax-rule (with-json-result str exp ...)
   (with-http-server `((200 ,str))
     (parameterize ((%swh-base-url (%local-url)))
@@ -98,6 +109,14 @@ (define-syntax-rule (with-json-result str exp ...)
                  (directory-entry-length entry)))
          (lookup-directory "123"))))
 
+(test-equal "lookup-directory-by-nar-hash"
+  "swh:1:dir:84a8b34591712c0a90bab0af604188bcd1fe3153"
+  (with-json-result %external-id
+    (lookup-directory-by-nar-hash
+     (nix-base32-string->bytevector
+      "0qqygvlpz63phdi2p5p8ncp80dci230qfa3pwds8yfxqqaablmhb")
+     'sha256)))
+
 (test-equal "rate limit reached"
   3000000000
   (let ((too-many (build-response