diff --git a/libagent/device/trezor.py b/libagent/device/trezor.py
index b0abcfe..806f455 100644
--- a/libagent/device/trezor.py
+++ b/libagent/device/trezor.py
@@ -6,6 +6,7 @@ import os
 import subprocess
 import sys
 
+import mnemonic
 import semver
 
 from . import interface
@@ -81,6 +82,13 @@ class Trezor(interface.Device):
             if _is_open_tty(sys.stdin):
                 return cli_handler(msg)  # CLI-based PIN handler
 
+            # Reusing environment variable from trezorlib/client.py
+            passphrase = os.getenv("PASSPHRASE")
+            if passphrase is not None:
+                log.info("Using PASSPHRASE environment variable.")
+                return self._defs.PassphraseAck(
+                    passphrase=mnemonic.Mnemonic.normalize_string(passphrase))
+
             passphrase = _message_box('Please enter passphrase:')
             return self._defs.PassphraseAck(passphrase=passphrase)
 
diff --git a/setup.py b/setup.py
index 8c507b9..f5e72d2 100755
--- a/setup.py
+++ b/setup.py
@@ -20,6 +20,7 @@ setup(
         'python-daemon>=2.1.2',
         'ecdsa>=0.13',
         'ed25519>=1.4',
+        'mnemonic>=0.18',
         'pymsgbox>=1.0.6',
         'semver>=2.2',
         'unidecode>=0.4.20',