diff --git a/package/libdeflate/libdeflate.mk b/package/libdeflate/libdeflate.mk
index 67e97c6e5e..c09a033325 100644
--- a/package/libdeflate/libdeflate.mk
+++ b/package/libdeflate/libdeflate.mk
@@ -9,8 +9,17 @@ LIBDEFLATE_SITE = $(call github,ebiggers,libdeflate,v$(LIBDEFLATE_VERSION))
 LIBDEFLATE_LICENSE = MIT
 LIBDEFLATE_LICENSE_FILES = COPYING
 LIBDEFLATE_INSTALL_STAGING = YES
+
+LIBDEFLATE_CFLAGS = -D_DEFAULT_SOURCE
+
+# Thumb build is broken, build in ARM mode, since all architectures
+# that support Thumb1 also support ARM.
+ifeq ($(BR2_ARM_INSTRUCTIONS_THUMB),y)
+LIBDEFLATE_CFLAGS += -marm
+endif
+
 LIBDEFLATE_CONF_OPTS = \
-	-DCMAKE_C_FLAGS="$(TARGET_CFLAGS) -D_DEFAULT_SOURCE"
+	-DCMAKE_C_FLAGS="$(TARGET_CFLAGS) $(LIBDEFLATE_CFLAGS)"
 
 ifeq ($(BR2_SHARED_LIBS),y)
 LIBDEFLATE_CONF_OPTS += \