The victory will be a bittersweet one for the Pole, who weighed in seven pounds over the limit and was fined 50% of his purse.
Nevertheless, the 33-year-old is now on a three-fight winning run, having enjoyed earlier victories over Aleksandar Rakas and Michal Pietrzak.
Romanowski improved his record to 16 victories, eight defeats and one no-contest.
Elsewhere, Dawid Smielowski maintained his KSW undefeated record after landing a devastating body kick to secure a knockout victory over fellow countryman Patryk Kaczmarczyk.
In one of the most exciting fights of the night, Dutchman Brian Hooi introduced himself to the KSW audience with an emphatic knockout finish against Michal Pietrzak.
Frenchman Oumar Sy, in contrast, improved his record to 7-0 with a dominant decision victory over Bartlomiej Gladkowicz.
Former world kickboxing champion Emilia Czerwinska was also successful with an impressive win over Croatian starlet Sara Luzar-Smajic.
KSW 72 Results
Main Event: Welterweight
Tomasz Romanowski (16-8) def. Andrzej Grzebyk (18-6) via TKO (punches), round 1, 3:53.
Co-Main Event: Featherweight
Dawid Smielowski (10-0) def. Patryk Kaczmarczyk (8-2) via KO (body kick), round 2, 2:51.
Brian Hooi (18-8) def. Michal Pietrzak (10-6-1) via TKO (elbows), round 1, 3:20.
Emilia Czerwinska (2-0) def. Sara Luzar-Smajic (4-2) via unanimous decision (29-27, 29-28 x 2).
Oumar Sy (7-0) vs. Bartlomiej Gladkowicz (8-3) via unanimous decision (30-25, 30-26, 30-27).
Patryk Surdyn (7-2) def. Gustavo Oliveira (8-2) via unanimous decision (30-26 x 3).
Yann Liasse (8-1) def. Hubert Szymajda (9-6) via unanimous decision (30-26, 29-28 x 2).
Robert Maciejowski (6-4-1) def. Arkadiusz Kaszuba (3-2) via split decision (29-28 x 2, 28-29).