Mano Vosawai's first-half try and the boot of Kris Burton put the Italian outfit deservedly in front at the break, two penalties from Ruaridh Jackson the best the Warriors could muster.
But the introduction of man of the match Scott Wight was decisive for Glasgow and Seymour, who was named in the Scotland squad for the autumn internationals this week, was at his opportunistic best.
The 24-year-old winger struck twice in four minutes after the hour mark and Townsend was all smiles at the final whistle after watching his side seal their fifth successive win in the RaboDirect PRO12.
"We're delighted with the spirit of the players showed to pursue and claim a win against extremely tough opposition," he said.
"We didn't play as well as we wanted to in the first half and Treviso put us under pressure. But we really stepped up in the second half, and to a man we delivered.
"Tommy Seymour, Niko Matawalu and Scott Wight were all terrific, and we deserved the win in the end."
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