diff --git a/src/medley/core.cljc b/src/medley/core.cljc
index 304a83f..ae2fb33 100644
--- a/src/medley/core.cljc
+++ b/src/medley/core.cljc
@@ -40,6 +40,19 @@
 (defn- assoc-some-transient! [m k v]
   (if (nil? v) m (assoc! m k v)))
 
+(defn assoc-conj
+  "Associate a key with a non-nil value in a map. If the key already exists
+  in the map, a vector of values is associated with the key."
+  [map key val]
+  (if (nil? val)
+    map
+    (assoc map key
+           (if-let [cur (get map key)]
+             (if (vector? cur)
+               (conj cur val)
+               [cur val])
+             val))))
+
 (defn assoc-some
   "Associates a key k, with a value v in a map m, if and only if v is not nil."
   ([m k v]
diff --git a/test/medley/core_test.cljc b/test/medley/core_test.cljc
index ed46752..67eb040 100644
--- a/test/medley/core_test.cljc
+++ b/test/medley/core_test.cljc
@@ -24,6 +24,13 @@
   (is (= (m/dissoc-in {:a 1} [])
          {:a 1})))
 
+(deftest test-assoc-conj
+  (is (= (m/assoc-conj nil :a 1) {:a 1}))
+  (is (= (m/assoc-conj nil :a nil) nil))
+  (is (= (m/assoc-conj {:a nil} :a 1) {:a 1}))
+  (is (= (m/assoc-conj {:a 1} :a 2) {:a [1 2]}))
+  (is (= (m/assoc-conj {:a 1} :a nil) {:a 1})))
+
 (deftest test-assoc-some
   (is (= (m/assoc-some {:a 1} :b 2) {:a 1 :b 2}))
   (is (= (m/assoc-some {:a 1} :b nil) {:a 1}))