Raddysh scored his 21st goal to set a Lightning record for the most by a defenseman in a season. The previous mark of 20 was shared by Dan Boyle (2006-07) and Victor Hedman (2021-22).
Charle-Edouard D’Astous and Nikita Kucherov also scored for the Lightning (48-22-6), who have won four of their past five games and went 5-1-1 on a seven-game homestand. Andrei Vasilevskiy made 21 saves.
Earlier Saturday, Tampa Bay clinched a berth in the Stanley Cup Playoffs for the ninth consecutive season when the Detroit Red Wings lost 4-1 at the New York Rangers.
Casey Mittelstadt scored for the Bruins (43-26-8), who have lost two straight after a four-game winning streak. Jeremy Swayman made 20 saves.
Raddysh gave Tampa Bay a 2-1 lead at 14:29 with a wrist shot from a sharp angle in the right face-off circle that got under Swayman’s right shoulder.
Kucherov added an empty-net goal at 18:27 for the 3-1 final.
Mittelstadt gave Boston a 1-0 lead at 6:47 of the second period after Viktor Arvidsson won a puck battle along the right wall and found Mittelstadt open for a wrist shot from between the circles.
D’Astous tied it 1-1 at 2:13 of the third. After his initial shot from the top of the slot was blocked, he buried his own rebound from a sharp angle on the left side.