ESPN said Simmons' agent Rich Paul had confirmed the deal, although the parties had until mid-October to finalize the extension of Simmons' rookie contract.
Simmons, 22, was the number one overall draft pick in 2016 and after injury saw him miss the 2016-17 season he was named Rookie of the Year in 2017-18.
He averaged 16.9 points, 8.8 rebounds and 7.7 assists for the 76ers last season, when he was named an All-Star for the first time. In the past two seasons he has had 22 triple-doubles and 80 double-doubles, emerging as a dominant finisher at the rim.
He joined Hall of Famer Oscar Robertson as the only NBA players with at least 10 triple-doubles -- posting double figures in three key statistical categories -- in each of their first two seasons.
Last season the Sixers were knocked out of the Eastern Conference semi-finals for the second straight season.
The extension will come into effect in the 2020-21 season after the final year of his rookie deal.