The NBA formally announced the suspension Friday afternoon.
Smith tested positive for marijuana, a source with direct knowledge of the result told Yahoo Sports.
Smith declined comment through the National Basketball Players Association.
"There is a collectively-bargained process in place, established by the NBA and the Players Association, and until that process has played itself out, it would be inappropriate for the NBPA or J.R. to make further statements about the matter," Ron Klempner, the union's acting executive director, said in a statement. "We all look forward to a resolution in due course."
Smith signed a three-year, $18 million contract extension in July, and soon thereafter underwent knee surgery that threatens to cost him part of the Knicks' season.
Smith, 27, averaged a career-best 18.1 points a game for the Knicks last season. The lingering knee issues appeared to affect his play in the postseason, especially in the Eastern Conference semifinal loss to the Indiana Pacers. Smith shot only 29 percent in the six-game series defeat to Indiana.
Smith's career has been checkered with issues, including a 90-day prison stay for causing an accident that killed a passenger in his car in 2007. The NBA suspended Smith seven games in 2009 for his role in the crash.
Smith has also had several other suspensions, including one for his part in a 2006 Denver Nuggets-New York Knicks brawl (10 games) and a 2007 nightclub incident (three games).
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