diff --git a/lens.lisp b/lens.lisp
index 5596a9b..42044d0 100644
--- a/lens.lisp
+++ b/lens.lisp
@@ -423,6 +423,26 @@
                       (reverse (cdr it))))
               (alexandria:hash-table-alist groups)))))
 
+(defun hash-join (probe join-fn &key (test 'eql) (key 'car))
+  (let* ((lookup (make-hash-table :test test :size (length probe)))
+         (lookup-fn (functionalize lookup)))
+    (map nil
+         (lambda (it)
+           (setf (gethash (funcall key it)
+                          lookup)
+                 it))
+         probe)
+    (lambda (collection)
+      (map (etypecase collection
+             (list 'list)
+             (vector 'vector)
+             (sequence 'list))
+           (lambda (it)
+             (let* ((key-value (funcall key it))
+                    (matching-probe (funcall lookup-fn key-value)))
+               (funcall join-fn it matching-probe)))
+           collection))))
+
 #+nil
 (defmacro <> (arity &rest funs)
   (let ((arg-syms (loop repeat arity collect (gensym))))
diff --git a/package.lisp b/package.lisp
index 17fc338..aa01b33 100644
--- a/package.lisp
+++ b/package.lisp
@@ -22,6 +22,7 @@
            #:∘ #:suffixp #:functionalize #:inc #:group-by #:keys
            #:conj #:disj #:delay #:of-type #:transform #:calling*
            #:calling
+           #:hash-join
            #:tap))
 
 (defpackage :data-lens.transducers.internals