The two signed four-year contracts that will be worth a combined $52 million, Paul Coro of the Arizona Republic reports. Markieff will earn $32 million, while Marcus will make $20 million.
Markieff averaged 13.8 points and six rebounds last season for the Suns, and Marcus, who was traded to Phoenix in February 2013 from the Houston Rockets, averaged 9.7 points and 3.9 rebounds in 82 games. The brothers played together in college at Kansas and were the 13th and 14th picks in the 2014 draft.
ESPN's Marc Stein originally reported the twins were close to an agreement on Monday.
Last week the Suns signed point guard Eric Bledsoe to a five-year, $70 million contract after a summer stalemate.
Phoenix opens preseason play on Oct. 8 against Brazilian team Flamengo. The Suns start the regular season Oct. 29 when they host the Los Angeles Lakers.
- Scooby Axson