The experienced midfielder, 32, was released by Barclays Premier League club QPR at the start of this week and has now linked up with the U's initially until the end of the season.
Rowlands joined the R's from Brentford in 2003 and went on to make well over 200 appearances for the Loftus Road club, becoming their longest-serving player prior to his release.
The former Republic of Ireland international is available to make his debut in Saturday's home clash against Sheffield United.
Manager John Ward told the club's official website: "Martin has a wealth of experience and when we heard he was leaving QPR we moved quickly to get him across here. We had seen him play in a behind-closed-doors game recently and my ears pricked up when I heard he might be available.
"The move means we have got a bit more seniority in the squad now and he has ambitions to go beyond the end of the season with us."