diff --git a/lib/minikanren/relations.sx b/lib/minikanren/relations.sx
index 4d78f1e1..768dff92 100644
--- a/lib/minikanren/relations.sx
+++ b/lib/minikanren/relations.sx
@@ -81,3 +81,18 @@
     (conde
       ((nullo l) (nullo p))
       ((fresh (a d perm-d) (conso a d l) (permuteo d perm-d) (inserto a perm-d p))))))
+
+(define
+  flatteno
+  (fn
+    (tree flat)
+    (conde
+      ((nullo tree) (nullo flat))
+      ((pairo tree)
+        (fresh
+          (h t hf tf)
+          (conso h t tree)
+          (flatteno h hf)
+          (flatteno t tf)
+          (appendo hf tf flat)))
+      ((nafc (nullo tree)) (nafc (pairo tree)) (== flat (list tree))))))
diff --git a/lib/minikanren/tests/flatteno.sx b/lib/minikanren/tests/flatteno.sx
new file mode 100644
index 00000000..4a01780a
--- /dev/null
+++ b/lib/minikanren/tests/flatteno.sx
@@ -0,0 +1,42 @@
+(mk-test "flatteno-empty" (run* q (flatteno (list) q)) (list (list)))
+
+(mk-test
+  "flatteno-atom"
+  (run* q (flatteno 5 q))
+  (list (list 5)))
+
+(mk-test
+  "flatteno-flat-list"
+  (run* q (flatteno (list 1 2 3) q))
+  (list (list 1 2 3)))
+
+(mk-test
+  "flatteno-singleton"
+  (run* q (flatteno (list 1) q))
+  (list (list 1)))
+
+(mk-test
+  "flatteno-nested-once"
+  (run*
+    q
+    (flatteno (list 1 (list 2 3) 4) q))
+  (list (list 1 2 3 4)))
+
+(mk-test
+  "flatteno-nested-twice"
+  (run*
+    q
+    (flatteno
+      (list
+        1
+        (list 2 (list 3 4))
+        5)
+      q))
+  (list (list 1 2 3 4 5)))
+
+(mk-test
+  "flatteno-keywords"
+  (run* q (flatteno (list :a (list :b :c) :d) q))
+  (list (list :a :b :c :d)))
+
+(mk-tests-run!)
diff --git a/plans/minikanren-on-sx.md b/plans/minikanren-on-sx.md
index 4151863d..1b71ac0f 100644
--- a/plans/minikanren-on-sx.md
+++ b/plans/minikanren-on-sx.md
@@ -111,7 +111,11 @@ Key semantic mappings:
 - [x] `caro` / `cdro` / `conso` / `firsto` / `resto`
 - [x] `reverseo` `l` `r` — reverse of list. Forward is fast; backward is `run 1`-clean,
       `run*` diverges due to interleaved unbounded list search (canonical TRS issue).
-- [ ] `flatteno` `l` `f` — flatten nested lists (deferred — needs atom predicate)
+- [x] `flatteno` `l` `f` — flatten nested lists. Three conde clauses:
+      empty → empty; pair → recurse on car & cdr + appendo; otherwise atom →
+      `(== flat (list tree))`. Atom-ness is asserted via `nafc (nullo tree)
+      + nafc (pairo tree)` — both succeed only when tree is a ground non-list.
+      Works on ground inputs.
 - [x] `permuteo` `l` `p` — permutation of list. Built on `inserto` (insert at any
       position) and recursive permutation of tail. All 6 perms of (1 2 3) generated
       forward; backward `run 1 q (permuteo q (a b c))` finds the input.
@@ -169,6 +173,11 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-08** — **flatteno**: nested-list flattener via conde + appendo.
+  Atom-ness is detected via `nafc (nullo ...) + nafc (pairo ...)` so the
+  third clause only fires when the input is a ground non-list. Works for
+  `()` / atoms / flat lists / arbitrarily-nested lists. 7 new tests,
+  253/253 cumulative. Phase 4 list relations all complete.
 - **2026-05-08** — **Laziness tests**: explicitly verifies the
   Zzz-on-conde-clauses + mk-mplus-swap-on-paused machinery: an
   infinitely-recursive relation truncated via `run 4 q (listo-aux q)`