diff --git a/tests/api/test_eccsi.c b/tests/api/test_eccsi.c
index df09a80e2b2..1a2b3ff48da 100644
--- a/tests/api/test_eccsi.c
+++ b/tests/api/test_eccsi.c
@@ -860,3 +860,192 @@ int test_wc_Eccsi_FeatureCoverage(void)
 #endif
     return EXPECT_RESULT();
 }
+
+/* wc_FreeEccsiKey() must wipe the SSK before releasing it. Only the heap math
+ * backend keeps mp_int digits in a separate allocation, so only there can a
+ * free hook see what is handed back to the allocator. WOLFSSL_CHECK_MEM_ZERO
+ * and WOLFSSL_MEMORY_LOG put a header in front of each allocation, so the free
+ * hook would never be handed key.ssk.dp itself. Platforms whose XFREE skips
+ * the hooks entirely are detected at run time by eccsi_zero_probe(). */
+#if defined(WOLFCRYPT_HAVE_ECCSI) && defined(WOLFCRYPT_ECCSI_CLIENT) && \
+    defined(USE_INTEGER_HEAP_MATH) && defined(USE_WOLFSSL_MEMORY) && \
+    !defined(WOLFSSL_NO_MALLOC) && !defined(WOLFSSL_STATIC_MEMORY) && \
+    !defined(WOLFSSL_CHECK_MEM_ZERO) && !defined(WOLFSSL_MEMORY_LOG)
+#define TEST_ECCSI_FREE_ZEROIZE
+
+/* These callback types take (func, line) too under WOLFSSL_DEBUG_MEMORY. */
+#ifdef WOLFSSL_DEBUG_MEMORY
+    #define ECCSI_ZERO_CB_TAIL   , const char* func, unsigned int line
+    #define ECCSI_ZERO_CB_ARGS   , func, line
+#else
+    #define ECCSI_ZERO_CB_TAIL
+    #define ECCSI_ZERO_CB_ARGS
+#endif
+
+/* The hooks delegate to the allocators that were installed before the test
+ * (e.g. WOLFSSL_TRACK_MEMORY's), so blocks that outlive the test, such as
+ * cached ECC curve values, can still be freed by them later. */
+static wolfSSL_Malloc_cb  eccsi_zero_prev_mc;
+static wolfSSL_Free_cb    eccsi_zero_prev_fc;
+static wolfSSL_Realloc_cb eccsi_zero_prev_rc;
+
+/* State for eccsi_zero_probe(). */
+static int         eccsi_zero_probing;
+static const void* eccsi_zero_probe_alloc;
+static int         eccsi_zero_probe_freed;
+
+static const void* eccsi_zero_watch;
+static size_t      eccsi_zero_watchSz;
+static int         eccsi_zero_seen;
+static int         eccsi_zero_clean;
+
+static void* eccsi_zero_malloc_cb(size_t size ECCSI_ZERO_CB_TAIL)
+{
+    void* p;
+
+    if (eccsi_zero_prev_mc != NULL) {
+        p = eccsi_zero_prev_mc(size ECCSI_ZERO_CB_ARGS);
+    }
+    else {
+        p = malloc(size);
+    }
+    if (eccsi_zero_probing && (eccsi_zero_probe_alloc == NULL)) {
+        eccsi_zero_probe_alloc = p;
+    }
+    return p;
+}
+
+static void eccsi_zero_free_cb(void* ptr ECCSI_ZERO_CB_TAIL)
+{
+    if (eccsi_zero_probing && (ptr != NULL) &&
+            (ptr == eccsi_zero_probe_alloc)) {
+        eccsi_zero_probe_freed = 1;
+    }
+    if ((ptr != NULL) && (ptr == eccsi_zero_watch)) {
+        const byte* p = (const byte*)ptr;
+        size_t i;
+        byte acc = 0;
+
+        for (i = 0; i < eccsi_zero_watchSz; i++) {
+            acc |= p[i];
+        }
+        eccsi_zero_seen = 1;
+        eccsi_zero_clean = (acc == 0);
+    }
+    if (eccsi_zero_prev_fc != NULL) {
+        eccsi_zero_prev_fc(ptr ECCSI_ZERO_CB_ARGS);
+    }
+    else {
+        free(ptr);
+    }
+}
+
+static void* eccsi_zero_realloc_cb(void* ptr, size_t size ECCSI_ZERO_CB_TAIL)
+{
+    if (eccsi_zero_prev_rc != NULL) {
+        return eccsi_zero_prev_rc(ptr, size ECCSI_ZERO_CB_ARGS);
+    }
+    return realloc(ptr, size);
+}
+
+/* Checks that XMALLOC()/XFREE() reach the hooks with the pointer unchanged.
+ * Returns 1 when they do and 0 when both bypass the hooks, as on platforms
+ * that map them to an OS or pool allocator (see types.h); the wipe cannot be
+ * observed there. Returns -1 when the hooks are reached but see a different
+ * pointer, or only one of the pair reaches them: that is a failure. */
+static int eccsi_zero_probe(void)
+{
+    void* probe;
+    int ret;
+
+    eccsi_zero_probe_alloc = NULL;
+    eccsi_zero_probe_freed = 0;
+    eccsi_zero_probing = 1;
+    probe = XMALLOC(8, NULL, DYNAMIC_TYPE_TMP_BUFFER);
+    if (probe == NULL) {
+        ret = -1;
+    }
+    else {
+        ret = (eccsi_zero_probe_alloc == probe) ? 1 :
+              (eccsi_zero_probe_alloc == NULL) ? 0 : -1;
+        XFREE(probe, NULL, DYNAMIC_TYPE_TMP_BUFFER);
+        if (eccsi_zero_probe_freed != ret) {
+            ret = -1;
+        }
+    }
+    eccsi_zero_probing = 0;
+    return ret;
+}
+#endif
+
+int test_wc_Eccsi_FreeZeroizesSsk(void)
+{
+    EXPECT_DECLS;
+#if defined(TEST_ECCSI_FREE_ZEROIZE) && defined(USE_WOLFSSL_MEMORY)
+    EccsiKey key;
+    WC_RNG rng;
+    mp_int ssk;
+    ecc_point* pvt = NULL;
+    char mail[] = "test@wolfssl.com";
+    int hooked = 0;
+
+    XMEMSET(&key, 0, sizeof(key));
+    XMEMSET(&rng, 0, sizeof(rng));
+    XMEMSET(&ssk, 0, sizeof(ssk));
+    eccsi_zero_watch = NULL;
+    eccsi_zero_watchSz = 0;
+    eccsi_zero_seen = 0;
+    eccsi_zero_clean = 0;
+    eccsi_zero_prev_mc = NULL;
+    eccsi_zero_prev_fc = NULL;
+    eccsi_zero_prev_rc = NULL;
+
+    ExpectIntEQ(wolfSSL_GetAllocators(&eccsi_zero_prev_mc,
+        &eccsi_zero_prev_fc, &eccsi_zero_prev_rc), 0);
+    ExpectIntEQ(wolfSSL_SetAllocators(eccsi_zero_malloc_cb,
+        eccsi_zero_free_cb, eccsi_zero_realloc_cb), 0);
+    if (EXPECT_SUCCESS()) {
+        hooked = eccsi_zero_probe();
+        ExpectIntGE(hooked, 0);
+    }
+
+    ExpectIntEQ(wc_InitRng(&rng), 0);
+    ExpectNotNull(pvt = wc_ecc_new_point());
+    ExpectIntEQ(mp_init(&ssk), MP_OKAY);
+    ExpectIntEQ(wc_InitEccsiKey(&key, HEAP_HINT, INVALID_DEVID), 0);
+    ExpectIntEQ(wc_MakeEccsiKey(&key, &rng), 0);
+    ExpectIntEQ(wc_MakeEccsiPair(&key, &rng, WC_HASH_TYPE_SHA256,
+        (byte*)mail, (word32)XSTRLEN(mail), &ssk, pvt), 0);
+    ExpectIntEQ(wc_SetEccsiPair(&key, &ssk, pvt), 0);
+
+    /* The key's copy of the SSK lives in its own digit allocation. */
+    ExpectFalse(mp_iszero(&key.ssk));
+    ExpectNotNull(key.ssk.dp);
+    if (EXPECT_SUCCESS()) {
+        eccsi_zero_watch = key.ssk.dp;
+        eccsi_zero_watchSz = (size_t)key.ssk.used * sizeof(mp_digit);
+    }
+
+    wc_FreeEccsiKey(&key);
+    eccsi_zero_watch = NULL;
+
+    if (hooked == 1) {
+        ExpectIntEQ(eccsi_zero_seen, 1);
+        ExpectIntEQ(eccsi_zero_clean, 1);
+    }
+
+    if (pvt != NULL) {
+        wc_ecc_del_point(pvt);
+    }
+    mp_forcezero(&ssk);
+    wc_FreeRng(&rng);
+    (void)wolfSSL_SetAllocators(eccsi_zero_prev_mc, eccsi_zero_prev_fc,
+        eccsi_zero_prev_rc);
+    /* Report the test as skipped, not passed, where XMALLOC() and XFREE()
+     * bypass the hooks. */
+    if ((hooked == 0) && EXPECT_SUCCESS()) {
+        return TEST_SKIPPED;
+    }
+#endif
+    return EXPECT_RESULT();
+}
diff --git a/tests/api/test_eccsi.h b/tests/api/test_eccsi.h
index 80ec3257944..a4976fbeb85 100644
--- a/tests/api/test_eccsi.h
+++ b/tests/api/test_eccsi.h
@@ -26,9 +26,11 @@
 
 int test_wc_Eccsi_DecisionCoverage(void);
 int test_wc_Eccsi_FeatureCoverage(void);
+int test_wc_Eccsi_FreeZeroizesSsk(void);
 
 #define TEST_ECCSI_DECLS                                          \
     TEST_DECL_GROUP("eccsi", test_wc_Eccsi_DecisionCoverage),     \
-    TEST_DECL_GROUP("eccsi", test_wc_Eccsi_FeatureCoverage)
+    TEST_DECL_GROUP("eccsi", test_wc_Eccsi_FeatureCoverage),      \
+    TEST_DECL_GROUP("eccsi", test_wc_Eccsi_FreeZeroizesSsk)
 
 #endif /* WOLFCRYPT_TEST_ECCSI_H */
diff --git a/wolfcrypt/src/eccsi.c b/wolfcrypt/src/eccsi.c
index 95cab2236cf..e62445e926a 100644
--- a/wolfcrypt/src/eccsi.c
+++ b/wolfcrypt/src/eccsi.c
@@ -116,6 +116,11 @@ int wc_InitEccsiKey_ex(EccsiKey* key, int keySz, int curveId, void* heap,
 #endif
                 );
     }
+#if defined(WOLFCRYPT_ECCSI_CLIENT) && defined(WOLFSSL_CHECK_MEM_ZERO)
+    if (err == 0) {
+        mp_memzero_add("ECCSI ssk", &key->ssk);
+    }
+#endif
     if (err == 0) {
         err = wc_ecc_set_curve(&key->ecc, keySz, curveId);
     }
@@ -164,7 +169,10 @@ void wc_FreeEccsiKey(EccsiKey* key)
 
         wc_ecc_del_point_h(params->base, key->heap);
 #ifdef WOLFCRYPT_ECCSI_CLIENT
-        mp_free(&key->ssk);
+        mp_forcezero(&key->ssk);
+    #ifdef WOLFSSL_CHECK_MEM_ZERO
+        mp_memzero_check(&key->ssk);
+    #endif
         mp_free(&key->tmp);
         mp_free(&params->prime);
         mp_free(&params->b);