He has signed in time to possibly line-up for Rangers on Friday's SKY live game at Loftus Road against Peterborough United.
Johnson is match fit after playing for Stoke City for a period in the last few months before being sent-off and released from his contract.
He is highly recognised by Watford fans for being a big part of there promtion campaign from Division 2 to the Premiership where he was present in the Hornets side.
Johnson describes himself as a "hard working player who likes to keep things simple" and is looking forward to joining Rangers and meeting up with some former team-mates.
"At this time in the season it is the results that count not the performances. You have to scrap, battle and fight for everything."
"Its nice to come in and know so many faces from my Watford days, I also no Kevin Gallen and Birchy."
"I'v been training so I'm in pretty good shape, I'v missed the last 3 games because I was sent-off for Stoke against Wimbledon but I feel OK and can't wait to get stuck in."
It is thought if Johnson does start against Posh on Friday it will be Stevie Palmer who will make way for him.