diff --git a/src/lib/Hydra/Schema/Users.pm b/src/lib/Hydra/Schema/Users.pm
index 55f0f1cb..72686367 100644
--- a/src/lib/Hydra/Schema/Users.pm
+++ b/src/lib/Hydra/Schema/Users.pm
@@ -238,6 +238,12 @@ sub check_password {
             $self->setPassword($password);
         }
 
+        return 1;
+    } elsif ($authenticator->verify_password(sha1_hex($password), $self->password)) {
+        # The user's database record has their old password as sha1, re-hashed as Argon2.
+        # Store their password hashed only with Argon2.
+        $self->setPassword($password);
+
         return 1;
     } else {
         return 0;
diff --git a/t/Schema/Users.t b/t/Schema/Users.t
index 5f31af76..d8cbaf8c 100644
--- a/t/Schema/Users.t
+++ b/t/Schema/Users.t
@@ -39,4 +39,15 @@ ok($user->check_password("foobar"), "Checking the password, foobar, is right");
 isnt($user->password, "8843d7f92416211de9ebb963ff4ce28125932878", "The user has had their password rehashed.");
 ok($user->check_password("foobar"), "Checking the password, foobar, is still right");
 
+# All sha1 passwords will be upgraded when `hydra-init` is run, by passing the sha1 through
+# Argon2. Verify a rehashed sha1 validates too. This removes very weak password hashes
+# from the database without requiring users to log in.
+subtest "Hashing their sha1 as Argon2 still lets them log in with their password" => sub {
+    $user->setPassword("8843d7f92416211de9ebb963ff4ce28125932878"); # SHA1 of "foobar"
+    my $hashedHashPassword = $user->password;
+    isnt($user->password, "8843d7f92416211de9ebb963ff4ce28125932878", "The user has had their password's hash rehashed.");
+    ok($user->check_password("foobar"), "Checking the password, foobar, is still right");
+    isnt($user->password, $hashedHashPassword, "The user's hashed hash was replaced with just Argon2.");
+};
+
 done_testing;