Chasing 231 to win, Bengal was given a blistering start by opener Arindam Das (59 off 62 balls) and SP Goswami (45 off 59 balls), who stitched together 111 runs in 19 overs.
Karnataka's Udit Patel tried to stop the run flow, removing both the openers and reducing the rivals to 116 for 20.5 overs.
However, first down Tiwary soon took control of the match and even through he kept losing wickets at the other end, he in company of Wriddhiman Saha (36 off 101 balls) helped Bengal surpass the target in the 49th over.
Earlier, AB Dinda (2/50), LR Shukla (2/52) and SL Pakre (2/34) helped Bengal restrict Karnataka at 230 for nine.