Seeley battled it out with Davey Todd (Milenco by Padgett’s Honda) throughout, before finally making a break for it on the fourth and final lap after Jeremy McWilliams put a move on Todd at York Corner, with both riders going slightly wide.
Todd forced his way back into second place but he was unable to close the deficit to Seeley, who notched up his 13th win in the class on the IFS Yamaha R6.
Todd – a Supersport winner in the rain in 2019 – finished as the runner-up, 1.9s back, with 58-year-old McWilliams delivering a dream podium for Northern Ireland’s Burrows Engineering/RK Racing team.
Adam McLean finished fourth on the McAdoo Kawasaki after losing his chance of a rostrum when he made a mistake at the start/finish chicane.
Todd’s team-mate Conor Cummins was fifth with the top six completed by Dean Harrison on the DAO Racing Kawasaki.
Michael Dunlop was a retirement after the first lap on his MD Racing Yamaha.
The race was red-flagged on the second lap after a rider slid off at Magherabuoy chicane, with the re-run held over four laps instead of the scheduled five-lap distance.
1 A Seeley (Yamaha)
2 D Todd (Honda) +1.984s
3 J McWilliams (Yamaha) +5.976s
4 A McLean (Kawasaki) +15.399s
5 C Cummins (Honda) +24.689s
6 D Harrison (Kawasaki) +30.317s
7 J Hillier (Yamaha) +41.838s
8 M Rees (Kawasaki) +41.838s