Williams was selected 22nd overall in the 2006 NBA draft by the New Jersey Nets, using the pick they got from the Denver Nuggets in a trade for Kenyon Martin. Former teammates Josh Boone, Rudy Gay, Hilton Armstrong, and Denham Brown were also selected, with all but Brown being first-round picks. Marcus Williams was named to the Rookie team for the 2007 T-Mobile Rookie Challenge at the 2007 NBA All-Star Weekend. As a rookie in 2006–07, Williams played in 79 games, averaging 6.8 ppg and 3.3 apg.