Millsap becomes the latest former All-Star to join the Nets over the past few seasons, joining Blake Griffin, Kevin Durant, Kyrie Irving and James Harden. Millsap's four All-Star appearances bring Brooklyn's total up to 38.
For the first time since his second year in the league in 2007-08, Millsap failed to average double figures in scoring last season. He averaged just 9.0 points and 4.7 rebounds in 56 games for the Nuggets.
Following a trade for Aaron Gordon in late March, Millsap lost the starting job he had held with Denver since signing with the team back in 2017.
He averaged just 6.4 points and 3.9 rebounds in 12.1 minutes per game in the Nuggets' two playoff rounds this past postseason, a sign the team was moving away from the 36-year-old.
But Millsap still provided veteran experience and was a leader for the Nuggets' second unit, helping Denver's bench outplay Portland's in a first-round win over the Blazers.
Millsap will help solidify the Nets' frontcourt rotation after the loss of Jeff Green, who signed with the Nuggets earlier this summer.
ESPN's Ohm Youngmisuk contributed to this report.