diff --git a/lib/minikanren/peano.sx b/lib/minikanren/peano.sx
new file mode 100644
index 00000000..9865cf72
--- /dev/null
+++ b/lib/minikanren/peano.sx
@@ -0,0 +1,35 @@
+;; lib/minikanren/peano.sx — Peano-encoded natural-number relations.
+;;
+;; Same encoding as `lengtho`: zero is the keyword `:z`; successors are
+;; `(:s n)`. So 3 = `(:s (:s (:s :z)))`. `(:z)` and `(:s ...)` are normal
+;; SX values that unify positionally — no special primitives needed.
+;;
+;; Peano arithmetic is the canonical miniKanren way to test addition /
+;; multiplication / less-than relationally without an FD constraint store.
+;; (CLP(FD) integers come in Phase 6.)
+
+(define zeroo (fn (n) (== n :z)))
+
+(define succ-of (fn (n m) (== m (list :s n))))
+
+(define
+  pluso
+  (fn
+    (a b c)
+    (conde
+      ((== a :z) (== b c))
+      ((fresh (a-1 c-1) (== a (list :s a-1)) (== c (list :s c-1)) (pluso a-1 b c-1))))))
+
+(define minuso (fn (a b c) (pluso b c a)))
+
+(define lteo (fn (a b) (fresh (k) (pluso a k b))))
+
+(define lto (fn (a b) (fresh (sa) (succ-of a sa) (lteo sa b))))
+
+(define
+  *o
+  (fn
+    (a b c)
+    (conde
+      ((== a :z) (== c :z))
+      ((fresh (a-1 ab-1) (== a (list :s a-1)) (*o a-1 b ab-1) (pluso b ab-1 c))))))
diff --git a/lib/minikanren/tests/peano.sx b/lib/minikanren/tests/peano.sx
new file mode 100644
index 00000000..682a3dd4
--- /dev/null
+++ b/lib/minikanren/tests/peano.sx
@@ -0,0 +1,119 @@
+;; lib/minikanren/tests/peano.sx — Peano arithmetic.
+;;
+;; Builds Peano numbers via a host-side helper so tests stay readable.
+;; (mk-nat 3) → (:s (:s (:s :z))).
+
+(define
+  mk-nat
+  (fn (n) (if (= n 0) :z (list :s (mk-nat (- n 1))))))
+
+;; --- zeroo ---
+
+(mk-test
+  "zeroo-zero-succeeds"
+  (run* q (zeroo :z))
+  (list (make-symbol "_.0")))
+(mk-test
+  "zeroo-non-zero-fails"
+  (run* q (zeroo (mk-nat 1)))
+  (list))
+
+;; --- pluso forward ---
+
+(mk-test
+  "pluso-forward-2-3"
+  (run* q (pluso (mk-nat 2) (mk-nat 3) q))
+  (list (mk-nat 5)))
+
+(mk-test "pluso-forward-zero-zero" (run* q (pluso :z :z q)) (list :z))
+
+(mk-test
+  "pluso-forward-zero-n"
+  (run* q (pluso :z (mk-nat 4) q))
+  (list (mk-nat 4)))
+
+(mk-test
+  "pluso-forward-n-zero"
+  (run* q (pluso (mk-nat 4) :z q))
+  (list (mk-nat 4)))
+
+;; --- pluso backward ---
+
+(mk-test
+  "pluso-recover-augend"
+  (run* q (pluso q (mk-nat 2) (mk-nat 5)))
+  (list (mk-nat 3)))
+
+(mk-test
+  "pluso-recover-addend"
+  (run* q (pluso (mk-nat 2) q (mk-nat 5)))
+  (list (mk-nat 3)))
+
+(mk-test
+  "pluso-enumerate-pairs-summing-to-3"
+  (run*
+    q
+    (fresh (a b) (pluso a b (mk-nat 3)) (== q (list a b))))
+  (list
+    (list :z (mk-nat 3))
+    (list (mk-nat 1) (mk-nat 2))
+    (list (mk-nat 2) (mk-nat 1))
+    (list (mk-nat 3) :z)))
+
+;; --- minuso ---
+
+(mk-test
+  "minuso-5-2-3"
+  (run* q (minuso (mk-nat 5) (mk-nat 2) q))
+  (list (mk-nat 3)))
+
+(mk-test
+  "minuso-n-n-zero"
+  (run* q (minuso (mk-nat 7) (mk-nat 7) q))
+  (list :z))
+
+;; --- *o ---
+
+(mk-test
+  "times-2-3"
+  (run* q (*o (mk-nat 2) (mk-nat 3) q))
+  (list (mk-nat 6)))
+
+(mk-test
+  "times-zero-anything-zero"
+  (run* q (*o :z (mk-nat 99) q))
+  (list :z))
+
+(mk-test
+  "times-3-4"
+  (run* q (*o (mk-nat 3) (mk-nat 4) q))
+  (list (mk-nat 12)))
+
+;; --- lteo / lto ---
+
+(mk-test
+  "lteo-success"
+  (run 1 q (lteo (mk-nat 2) (mk-nat 5)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "lteo-equal-success"
+  (run 1 q (lteo (mk-nat 3) (mk-nat 3)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "lteo-greater-fails"
+  (run* q (lteo (mk-nat 5) (mk-nat 2)))
+  (list))
+
+(mk-test
+  "lto-strict-success"
+  (run 1 q (lto (mk-nat 2) (mk-nat 5)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "lto-equal-fails"
+  (run* q (lto (mk-nat 3) (mk-nat 3)))
+  (list))
+
+(mk-tests-run!)
diff --git a/plans/minikanren-on-sx.md b/plans/minikanren-on-sx.md
index aa332a34..81c80da9 100644
--- a/plans/minikanren-on-sx.md
+++ b/plans/minikanren-on-sx.md
@@ -156,6 +156,11 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-07** — **Peano arithmetic** (`lib/minikanren/peano.sx`): zeroo, pluso,
+  minuso, lteo, lto, *o on Peano-encoded naturals (`:z` / `(:s n)`). pluso runs
+  forward, backward, and enumerates: `(run* q (fresh (a b) (pluso a b 3)
+  (== q (list a b))))` → all 4 pairs summing to 3. *o uses repeated pluso —
+  works for small inputs, slower for larger. 19 new tests, 188/188 cumulative.
 - **2026-05-07** — **Phase 5 piece A — conda**: soft-cut. Mirrors `condu` minus
   the `onceo` on the head: all head answers are conjuncted through the rest of
   the chosen clause. 7 new tests including the conda-vs-condu divergence test.