Roberson has played 36 career NBA games with Memphis and Golden State, but spent last season in Turkey and Israel. The Knicks signed him to a deal after he averaged 14.8 points in five games for their summer league team.
Terms of the contract were not disclosed.
Roberson's addition means the Knicks will have to drop one player before the season starts, and speculation has been that player could be Stephon Marbury. The veteran point guard has a year and more than $21 million left on his contract, but angered the organization last season with his actions during an injury-plagued season.
New York previously signed point guard Chris Duhon to a two-year deal earlier this summer. Knicks president Donnie Walsh said at the time that had no bearing on Marbury's future and he was in no rush to make any decisions.