Smith, who won the NBA dunking title a year ago and is known more for his athleticism, made a jumper with 33.9 seconds left in overtime to give the Hawks a 104-100 lead. He also scored on a jumper with 4.9 seconds to play in regulation to tie it at 94.
"Coach has put confidence in me. He knows I've been working on my jump shot, so he doesn't have any problem with me taking it," Smith said.
Smith, a second-year forward, was 6-of-12 from the field and finished with 19 points, nine rebounds and four blocks.
"Last year we would cringe when he would shoot the basketball," Atlanta coach Mike Woodson said. "Now, his shot looks like a shot and he is handling the ball a little better."
Vince Carter attempted a long 3-pointer in the final seconds of overtime but it rimmed out and Nenad Krstic also missed a follow attempt for the Atlantic Division-leading Nets, who have lost two straight.
"We have great trust in Vince's judgment," New Jersey coach Lawrence Frank said. "He wanted to put it on his shoulders and win it. It went in and out. We gave ourselves an opportunity for an offensive rebounds. The basketball gods saw it another way."
Carter had an earlier chance to win it for the Nets but fumbled the inbounds pass, and his desperation shot from about 20 feet at the buzzer wasn't even close.
"I just lost the ball," Carter said.
Carter scored seconds after Smith's clutch jumper in overtime to cut it to 104-102 and the Nets got a chance to win it when the Hawks failed to get off a shot within 24 seconds, giving New Jersey the ball with 7.1 seconds left.
"[Assistant coach] Larry Drew kept telling me to shoot my shot because it's right on target," Smith said. "The opportunity came, Al [Harrington] swung the ball to me, and I knocked down the jump shot."
Richard Jefferson and Carter scored 22 points apiece for New Jersey. Krstic had 21 points and Jason Kidd had 19 points, a season-high 17 assists and nine rebounds.
"You have to give Josh Smith credit. He made two huge shots," Frank said.
The Nets had taken the lead in regulation at 93-92 on a basket by Krstic with 1:44 left. The Hawks had a chance to go ahead, but Johnson had a layup attempt blocked by Krstic with about 27 seconds left. Kidd then hit one of two free throws for a 94-92 lead with 13.9 seconds to play.
Joe Johnson also finished with 19 points for the Hawks and Zaza Pachulia had 14 points and 12 rebounds.
Atlanta led 73-70 after three periods after outscoring the Nets 24-13 in the second quarter to take a 46-37 halftime lead.
Jefferson scored nine points in the first quarter and the Nets led 24-22 after one.
Carter hurt his right hamstring Sunday in a 101-91 loss to Indiana and was questionable until game time. He played 44 minutes and was 9-of-24 from the field, including 3-of-10 on 3-point attempts.
Kidd has 71 career triple-doubles. ... Salim Stoudamire, suspended for two games by the team for insubordination, scored five straight points early in the second quarter, seconds after
entering the game. ... The Nets beat the Hawks 99-91 on Dec. 30 in New Jersey in the only meeting between the clubs before Monday night. Carter led New Jersey with 37 points. ... The Hawks had lost four in a row and 10 of 12 to the Nets. ... Nets F Clifford Robinson played in his 1,304th game, breaking a tie with Elvin Hayes for ninth on the NBA's career list.