diff --git a/src/update_flash.c b/src/update_flash.c
index c2a27e48..77bc0ed1 100644
--- a/src/update_flash.c
+++ b/src/update_flash.c
@@ -623,6 +623,13 @@ static int RAMFUNCTION wolfBoot_swap_and_final_erase(int resume)
     #   define DELTA_BLOCK_SIZE 1024
     #endif
 
+    /* The per-sector fill loop advances in DELTA_BLOCK_SIZE steps, so a
+     * sector that is not a multiple of the block size would be written
+     * past the one-sector SWAP partition and misalign the resume path. */
+    #if (WOLFBOOT_SECTOR_SIZE % DELTA_BLOCK_SIZE) != 0
+    #error "Delta update: WOLFBOOT_SECTOR_SIZE % DELTA_BLOCK_SIZE != 0"
+    #endif
+
 static inline uint32_t wb_delta_im2n(uint32_t val)
 {
 #ifdef BIG_ENDIAN_ORDER
diff --git a/tools/unit-tests/Makefile b/tools/unit-tests/Makefile
index c34d235e..234ccf77 100644
--- a/tools/unit-tests/Makefile
+++ b/tools/unit-tests/Makefile
@@ -221,6 +221,7 @@ run: $(TESTS)
 	python3 unit-sign-delta-tlv.py || exit 1
 	python3 unit-sign-delta-cert-inv-off.py || exit 1
 	python3 unit-sign-delta-basehash-cleanup.py || exit 1
+	python3 unit-delta-sector-align.py || exit 1
 	python3 unit-sign-custom-tlv-le.py || exit 1
 	python3 unit-sign-custom-tlv-large.py || exit 1
 	python3 unit-sign-custom-tlv-pubkey-der.py || exit 1
@@ -264,6 +265,14 @@ unit-update-flash-delta:CFLAGS+=-DMOCK_PARTITIONS -DWOLFBOOT_NO_SIGN -DUNIT_TEST
 	-DWOLFBOOT_HASH_SHA256 -DPRINTF_ENABLED -DEXT_FLASH -DPART_UPDATE_EXT -DPART_SWAP_EXT \
 	-DDELTA_UPDATES -DDELTA_BLOCK_SIZE=512 -D__WOLFBOOT \
 	-DWOLFBOOT_ORIGIN=MOCK_ADDRESS_BOOT -DBOOTLOADER_PARTITION_SIZE=WOLFBOOT_PARTITION_SIZE
+# Intentionally misaligned config: the mock WOLFBOOT_SECTOR_SIZE (0x400) is
+# not a multiple of DELTA_BLOCK_SIZE (1536). Must fail to build with the
+# delta alignment #error. Used by unit-delta-sector-align.py only, so it is
+# not part of $(TESTS).
+unit-update-flash-delta-misalign:CFLAGS+=-DMOCK_PARTITIONS -DWOLFBOOT_NO_SIGN -DUNIT_TEST_AUTH \
+	-DWOLFBOOT_HASH_SHA256 -DPRINTF_ENABLED -DEXT_FLASH -DPART_UPDATE_EXT -DPART_SWAP_EXT \
+	-DDELTA_UPDATES -DDELTA_BLOCK_SIZE=1536 -D__WOLFBOOT \
+	-DWOLFBOOT_ORIGIN=MOCK_ADDRESS_BOOT -DBOOTLOADER_PARTITION_SIZE=WOLFBOOT_PARTITION_SIZE
 unit-update-flash-self-update:CFLAGS+=-DMOCK_PARTITIONS -DWOLFBOOT_NO_SIGN -DUNIT_TEST_AUTH \
 	-DWOLFBOOT_HASH_SHA256 -DPRINTF_ENABLED -DEXT_FLASH -DPART_UPDATE_EXT -DPART_SWAP_EXT \
 	-DRAM_CODE -DARCH_SIM -DUNIT_TEST_SELF_UPDATE_ONLY \
@@ -808,6 +817,10 @@ unit-update-flash-delta: ../../include/target.h unit-update-flash.c
 	gcc -o $@ unit-update-flash.c ../../src/image.c ../../src/delta.c \
 	$(WOLFBOOT_LIB_WOLFSSL)/wolfcrypt/src/sha256.c $(CFLAGS) $(LDFLAGS)
 
+unit-update-flash-delta-misalign: ../../include/target.h unit-update-flash.c
+	gcc -o $@ unit-update-flash.c ../../src/image.c ../../src/delta.c \
+	$(WOLFBOOT_LIB_WOLFSSL)/wolfcrypt/src/sha256.c $(CFLAGS) $(LDFLAGS)
+
 unit-update-flash-enc:CFLAGS+=-DMOCK_PARTITIONS -DWOLFBOOT_NO_SIGN -DUNIT_TEST_AUTH \
 	-DWOLFBOOT_HASH_SHA256 -DPRINTF_ENABLED -DEXT_FLASH -DPART_UPDATE_EXT \
 	-DPART_SWAP_EXT -DEXT_ENCRYPTED -DENCRYPT_WITH_CHACHA -DHAVE_CHACHA \
diff --git a/tools/unit-tests/unit-delta-sector-align.py b/tools/unit-tests/unit-delta-sector-align.py
new file mode 100644
index 00000000..644d0439
--- /dev/null
+++ b/tools/unit-tests/unit-delta-sector-align.py
@@ -0,0 +1,64 @@
+#!/usr/bin/env python3
+# unit-delta-sector-align.py
+#
+# Build-time invariant for DELTA_UPDATES: the per-sector fill loop in
+# wolfBoot_delta_update() (src/update_flash.c) calls wb_patch() with
+# DELTA_BLOCK_SIZE chunks until WOLFBOOT_SECTOR_SIZE bytes are produced, so
+# the reconstructed image is only sector-aligned when WOLFBOOT_SECTOR_SIZE is
+# a multiple of DELTA_BLOCK_SIZE. A mismatched config used to compile fine
+# and corrupt the delta apply (write past the one-sector SWAP partition,
+# misaligned resume); the build now rejects it with an #error.
+#
+# This test builds the real update_flash.c (via unit-update-flash.c) with a
+# deliberately misaligned DELTA_BLOCK_SIZE and asserts the build fails with
+# the invariant message.
+#
+# Copyright (C) 2026 wolfSSL Inc.
+#
+# This file is part of wolfBoot.
+#
+# wolfBoot is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# wolfBoot is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1335, USA
+
+import os
+import subprocess
+import sys
+
+MESSAGE = "WOLFBOOT_SECTOR_SIZE % DELTA_BLOCK_SIZE != 0"
+
+
+def main():
+    # The misalign target is not in $(TESTS) and its rule does not track
+    # src/update_flash.c (included via unit-update-flash.c), so drop any
+    # stale artifact to force a real rebuild.
+    try:
+        os.remove("unit-update-flash-delta-misalign")
+    except FileNotFoundError:
+        pass
+    p = subprocess.run(["make", "unit-update-flash-delta-misalign"],
+                       capture_output=True, text=True)
+    if p.returncode == 0:
+        print("FAIL: misaligned DELTA_BLOCK_SIZE config built, "
+              "expected the sector alignment #error")
+        return 1
+    if MESSAGE not in (p.stdout + p.stderr):
+        print("FAIL: build failed for the wrong reason:\n")
+        print(p.stderr[-2000:])
+        return 1
+    print("PASS: misaligned DELTA_BLOCK_SIZE rejected at build time")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())