Gardner, 30, initially moved to the KC Lightstream Stadium on a month’s loan and has impressed in the three appearances he’s made to date, leading to an extension.
Coach Craig Sandercock said: “Ade has been terrific for us while he’s been here. He’s the ultimate professional and has brought plenty of experience and talk to the side, both in training and on gameday.
“With the injuries we have in our outside backs at the moment it made sense to try and keep him here and I’m happy that we’ve managed to get that deal done.
“I’m sure Ade will continue to make a positive contribution to our club and is a great mentor for the younger players in our squad who will certainly benefit from having someone of his experience around the place.”
Gardner made 10 appearances for St Helens last season, scoring five tries.