After a quiet start Watford threatened first with an Allan Nielsen header from Neal Ardley's cross, but Andy Marshall made a good block.
Town went ahead after 23 minutes when Tommy Miller fired a 25-yarder into the top corner of the net.
Alec Chamberlain then pulled off a good block to deny Hermann Hreidarsson before plunging to his right to hold another Miller drive.
Ipswich's pressure paid off five minutes before the break with a fine second - Richard Naylor firing a 20-yard shot home after excellent work from strike partner Pablo Counago.
Watford squandered a chance to hit back when Tommy Smith trod on the ball as he led a three on one breakaway.
But he made amends five minutes after the break, racing on to Heidar Helguson's flick to coolly slot past Marshall.
Still Watford were not beaten and reduced the arrears again when skipper Neil Cox steered a fine header into the roof of the net from Ardley's 75th minute corner.
Clapham fired over on the break for Town, and a restless home crowd was only to rest easy when Bent pounced.
Bent held off Cox as a high ball was hammered forward, before turning to steer a left footed shot beyond Chamberlain.
Town flooded forward late on and only a good save from Chamberlain denied Fabian Wilnis a fifth goal, but Royle's men ran out deserving, if slightly edgy winners.