The decision was reached after Harden tested his right hamstring before tipoff at Barclays Center. He was upgraded from out to questionable a few hours before the pivotal matchup.
League sources told ESPN and The Athletic that Harden pushed for an opportunity to play and participated in the Nets' shootaround Tuesday morning. The All-Star guard noticed improvement in his right hamstring, prompting the team to upgrade his status.
Harden then tested his injured hamstring in pre-game warmups and didn't suffer any setbacks, clearing his path to play.
"I'm not sure the level of risk," Nets head coach Steve Nash said before the game. "I think it is James' decision. He wants to play. Ultimately, he wants to play. He's been pushing."
The 31-year-old Harden experienced right hamstring tightness when he attempted to lob a pass over Bucks guard Jrue Holiday and center Brook Lopez in the opening minute of the Nets' Game 1 win over Milwaukee on June 5. After lobbing the ball, Harden landed and immediately grabbed at his right hamstring.
He eventually left the floor and retreated to the Nets' locker room, and he remained out until his activation Tuesday.
The Nets are tied 2-2 in the Eastern Conference Semifinals with the Bucks and are preparing to be without star guard Kyrie Irving -- who sustained a right ankle sprain in Game 4 -- for the remainder of the second-round series, according to ESPN. The team has yet to rule Irving out beyond Game 5.
Game 5 of the best-of-seven series is scheduled for 8:30 p.m. EDT at Barclays Center in Brooklyn.