Hamsik grabbed a 31st-minute opener, turning in Camilo Zuniga's wayward shot, before his low cross set-up a Paolo Cannavaro tap-in just before half-time.
Catania were vastly improved after the break, and had a Gonzalo Bergessio header ruled out for a foul, but the hosts held on to draw level with Juve ahead of the champions' trip to Chievo on Sunday lunchtime.
Napoli were dominant in the opening period and set the tone with barely a minute gone when Goran Pandev beat two defenders before firing a powerful shot just over.
Edinson Cavani was also off target before his clever pass should have been turned in by Giandomenico Mesto only for the veteran full-back to miss the ball from close range.
Pablo Barrientos reminded Napoli of the away side's threat, the Sicilians began the day in seventh, when he shot over from outside the area on 28 minutes.
Napoli deservedly went ahead just after the half-hour thanks to Hamsik's goalscoring instincts.
Despite Zuniga cutting inside and lining up a shot, the Slovakia international continued his run to the back post and was rewarded when the ball scuffed into his path to side-foot high into the net.
Cavani then crashed a free-kick into the crossbar before Hamsik turned provider for Napoli's second.
Catania failed to properly clear a corner and Hamsik hooked the ball back across the six-yard box where Cannavaro, who was just played onside, was the first of three home players on hand to tuck the ball in.
The visitors could have grabbed a goal back two minutes after half-time only for Bergessio's header to be whistled off for a foul on Zuniga.
Catania continued to press, though, and Alejandro Gomez shot just wide after a one-two with Bergessio.
Hamsik was back in the thick of the action as he started a move that ended with Cavani heading a Mesto cross over.
Napoli were indebted to goalkeeper Morgan De Sanctis as he produced the best save of the match to keep out Francesco Lodi's free-kick on 66 minutes.
Lodi then curled wide after a scramble inside the area but it was not Catania's night as Barrientos was also narrowly off target with a placed shot late on.