Deng's decision comes after a report on Monday said the Heat expected Deng to opt out.
Deng signed a reported two-year, $20 million deal with the Heat last summer after previous stints with the Cleveland Cavaliers and Chicago Bulls. Over 72 games last season, the 30-year-old averaged 14 points, 5.2 rebounds and 1.9 assists per game while shooting 46.9% from the field.
By opting in, Deng will make $10.1 million during the upcoming season before testing free agent waters in 2016, when the salary cap is expected to spike because of the NBA's new television deal.
Heat point guard Goran Dragic has elected to opt out of his contract, along with veteran shooting guard Dwyane Wade. Wade would have made $16.1 million during the 2015–16 season if he had opted in.
Last season Miami went 37-45 and missed the playoffs.
Teams can begin negotiating with free agents on July 1, but contracts cannot be consummated until July 9.