Dany Heatley scored twice for the winners in the season finale for both teams.
Piros scored his first NHL goal 4:32 into the game, and completed the hat trick with a pair of second-period goals. In between, Ilya Kovalchuk added his 38th of the season to give Atlanta a quick 2-0 lead.
Heatley made it 5-2 with 3:10 to go in the third period, and added a power-play tally to extend his team record with 41 goals.
The Lightning had a team-record six-game road unbeaten streak snapped, and has dropped two in a row following an overall club-record 13-game unbeaten streak stopped. The Southeast Division champions had not lost back-to-back games since Feb. 4-6.
Regardless, Tampa Bay is the third seed in the Eastern Conference, and will play the Washington Capitals in the second playoff series in team history.
Atlanta ended the season with one loss in its last seven games (5-1-1), and was 19-14-5-1 after Bob Hartley was named coach on Jan. 14.