His previous spell with the club culminated in playing a leading role in Welsh's Championship winning season, before leaving to join the Blues last summer.
A graduate of the Newport Gwent Dragons Academy, Lewis gained representative honours for Wales at U18, U19 and U20 levels, and was top try scorer in the Wales U19 side that won the Grand Slam in 2006.
He made his debut for the Dragons against Munster at Musgrave Park in November 2007, and went onto make 17 appearances for the senior side. He also played for Ebbw Vale.
Called in to the Wales Sevens squad in February 2008 for the IRB Sevens tournaments in Wellington and San Diego, he and brother James became the first twins to represent Wales at senior level.
The scrum-halves' return to London Welsh will once again see him team up with James, who joined the Exiles in November 2010.
London Welsh head coach Lyn Jones said: "Rob played a huge part in the club's championship success and we welcome him back to London Welsh with open arms."
On re-joining London Welsh Rob Lewis said: "It's good to be back, I've missed the place and I've missed the boys. It's always been a club close to my heart.
"I've been following their results closely this season, and it's nice to be back to be a part of it once again. I'm not surprised by the start the club's made to the Aviva Premiership, there's always been a good core of boys here."