The Scotland international has been without a club since the autumn but did has spent time training with Championship side Halifax Panthers.
Walker, 26, made over 100 appearances for London between 2014 and 2019 before joining Wakefield on a two-year contract ahead of the 2020 campaign.
He was unable to establish a regular spot in the Trinity side and spent the second half of last season on loan at Featherstone Rovers, helping the club reach the Championship Grand Final.
The Essex-born player was not offered a new deal by Wakefield and did not return permanently to Rovers. He did join Halifax on trail in November but a permanent switch to the Shay Stadium did not materialise.
He has now returned to London, with a club statement confirming: "We are delighted to announce the signing of former player Alex Walker until the end of the season.
"The Scotland full-back will go straight into the squad for this weekend's Challenge Cup fixture against Bradford Bulls."