diff --git a/lightningd/lightningd.c b/lightningd/lightningd.c
index 2381557e4..2921fe996 100644
--- a/lightningd/lightningd.c
+++ b/lightningd/lightningd.c
@@ -75,6 +75,7 @@ static struct lightningd *new_lightningd(const tal_t *ctx,
 	htlc_in_map_init(&ld->htlcs_in);
 	htlc_out_map_init(&ld->htlcs_out);
 	ld->dev_disconnect_fd = -1;
+	ld->dev_hsm_seed = NULL;
 	ld->log_book = log_book;
 	ld->log = new_log(log_book, log_book, "lightningd(%u):", (int)getpid());
 
diff --git a/lightningd/lightningd.h b/lightningd/lightningd.h
index eff6cda6a..e0c6165d5 100644
--- a/lightningd/lightningd.h
+++ b/lightningd/lightningd.h
@@ -113,6 +113,9 @@ struct lightningd {
 	/* If we want to debug a subdaemon. */
 	const char *dev_debug_subdaemon;
 
+	/* If we want to set a specific non-random HSM seed. */
+	const u8 *dev_hsm_seed;
+
 	/* If we have a --dev-disconnect file */
 	int dev_disconnect_fd;
 
diff --git a/lightningd/options.c b/lightningd/options.c
index 816236c00..b147ed037 100644
--- a/lightningd/options.c
+++ b/lightningd/options.c
@@ -2,12 +2,14 @@
 #include <bitcoin/chainparams.h>
 #include <ccan/err/err.h>
 #include <ccan/opt/opt.h>
+#include <ccan/read_write_all/read_write_all.h>
 #include <ccan/short_types/short_types.h>
 #include <ccan/tal/grab_file/grab_file.h>
 #include <ccan/tal/str/str.h>
 #include <common/configdir.h>
 #include <common/version.h>
 #include <errno.h>
+#include <fcntl.h>
 #include <inttypes.h>
 #include <lightningd/bitcoind.h>
 #include <lightningd/chaintopology.h>
@@ -20,6 +22,8 @@
 #include <string.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
+#include <sys/stat.h>
+#include <sys/types.h>
 #include <sys/types.h>
 #include <unistd.h>
 #include <wire/wire.h>
@@ -253,6 +257,15 @@ static void config_register_opts(struct lightningd *ld)
 			       "regtest, or litecoin)");
 }
 
+static char *opt_set_hsm_seed(const char *arg, struct lightningd *ld)
+{
+	ld->dev_hsm_seed = tal_hexdata(ld, arg, strlen(arg));
+	if (ld->dev_hsm_seed)
+		return NULL;
+
+	return tal_fmt(NULL, "bad hex string '%s'", arg);
+}
+
 static void dev_register_opts(struct lightningd *ld)
 {
 	opt_register_noarg("--dev-no-broadcast", opt_set_bool,
@@ -266,6 +279,8 @@ static void dev_register_opts(struct lightningd *ld)
 			 "Time between gossip broadcasts in milliseconds (default: 30000)");
 	opt_register_arg("--dev-disconnect=<filename>", opt_subd_dev_disconnect,
 			 NULL, ld, "File containing disconnection points");
+	opt_register_arg("--dev-hsm-seed=<seed>", opt_set_hsm_seed,
+			 NULL, ld, "Hex-encoded seed for HSM");
 }
 
 static const struct config testnet_config = {
@@ -546,5 +561,18 @@ bool handle_opts(struct lightningd *ld, int argc, char *argv[])
 		errx(1, "no arguments accepted");
 
 	check_config(ld);
+
+	if (ld->dev_hsm_seed) {
+		int fd;
+		unlink("hsm_secret");
+		fd = open("hsm_secret", O_CREAT|O_WRONLY, 0400);
+		if (fd < 0 ||
+		    !write_all(fd, ld->dev_hsm_seed, tal_len(ld->dev_hsm_seed))
+		    || fsync(fd) != 0)
+			fatal("dev-hsm-seed: Could not write file: %s",
+			      strerror(errno));
+		close(fd);
+	}
+
 	return newdir;
 }
diff --git a/tests/utils.py b/tests/utils.py
index 395b86572..891a77ac6 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -225,6 +225,8 @@ class LightningD(TailableProc):
         TailableProc.__init__(self, lightning_dir)
         self.lightning_dir = lightning_dir
         self.port = port
+        # Last 32-bytes of final part of dir -> seed.
+        seed = (bytes(re.search('([^/]+)/*$', lightning_dir).group(1), encoding='utf-8') + bytes(32))[:32]
         self.cmd_line = [
             'lightningd/lightningd',
             '--bitcoin-datadir={}'.format(bitcoin_dir),
@@ -232,6 +234,7 @@ class LightningD(TailableProc):
             '--port={}'.format(port),
             '--network=regtest',
             '--dev-broadcast-interval=1000',
+            '--dev-hsm-seed={}'.format(seed.hex())
         ]
 
         self.cmd_line += ["--{}={}".format(k, v) for k, v in LIGHTNINGD_CONFIG.items()]