The 2003 World Cup winner spent over ten years with Wasps helping the club to four Premiership titles, two Heineken Cups, an Amlin Challenge Cup and two domestic Cup successes.
Since his retirement, the 34-year-old has been on an expedition to Mount Everest and been employed as a management consultant.
"This isn't a long term thing but after playing in a couple of recent charity games, Leon and Shaun (Edwards) contacted me to see if I could assist the club for the remainder of the season," said Lewsey.
"If they think I could positively contribute to my old club in any way- be it on the field or around the dressing room on match days, then I'd be happy to do so"
Interim director of rugby Leon Holden said: "This is a fantastic short term signing for us and I have no doubt Josh will make a huge contribution to the squad in what is left of the season. Josh has been playing some charity games lately and he's kept himself in remarkably good shape in the two years since he retired. We have had a few knocks to players in our back three in recent weeks - and with Tom Varndell out for a longer spell, we really do need some cover and we don't think we could have done any better than bringing someone like Josh back to the club."