The Lakers announced the extension on Wednesday.
Barnes averaged 6.7 points, 4.3 rebounds and 1.3 assists in 19.2 minutes over 53 games last season. He was signed by the Lakers as a free agent last July.
Barnes missed more than 25 games following right knee surgery in January.
Another member of the Lakers' bench, Shannon Brown, has yet to make a decision on his player option for next season worth $2.4 million.
"Not right now," Brown's agent Mark Bartelstein replied via text message when asked if there had been any progress made in Brown's decision-making process.
Brown, a 6-foot-4 shooting guard, averaged a career-high 8.7 points per game in 19.1 minutes per game as a substitute last season. The five-year veteran was one of six Lakers players to appear in all 82 regular season games in 2010-11.
Information from ESPNLosAngeles.com's Dave McMenamin and The Associated Press was used in this report.