Jaroslav Halak made his debut, though the Islanders didn't change too much in front of him from what they'd done in their two previous Barclays Center games this week.
It was another sluggish first period from the Islanders, followed by an onslaught. They rallied from three separate one-goal deficits over the first two periods and took the lead on Kyle Okposo's power move to the net with 15:42 to play.
The 6-3 win over the previously unbeaten Sharks was the Isles' third straight win in Brooklyn, all against Western Conference teams who hadn't lost prior to visiting the Islanders new home.
Halak made 22 saves, beaten by a deflection in close and a pair of no-chance goals on odd-man San Jose rushes. But the Islanders strafed Alex Stalock and the Sharks, who had given up just two goals in their first four games of the season.
Josh Bailey sprung Okposo for a semi-breakaway with the game 3-3 in the third, but Stalock turned aside Okposo's 20-foot shot. Okposo went to the end boards and retrieved the puck from Brenden Dillon, bulled to the front of the net and chipped a shot over Stalock for the go-ahead score.
Brock Nelson's wrister from the slot gave the Isles a two-goal lead with 8:50 to go. John Tavares had two more assists, giving him seven points in the last three games. Frans Nielsen put the game away with an empty-netter with 2:58 to go.
Halak led his team onto the Barclays Center ice for the first time this season, finally back after missing nearly all of the preseason and the opening four games with an upper-body injury.
"He was ready to play Thursday, but we wanted to make absolutely sure that there were no issues," Jack Capuano said after the morning skate. "He's a big part of our team, no question. I know he's ready to go."
Halak got a few tests in the opening period from a Sharks team that came in with four straight victories and only two goals allowed. But Halak's teammates, after a couple of good shifts to open the game, could not find much rhythm to their transition game and played too much of the first period in the Islanders zone.
That ended up costing them at 13:04 of the first. Joe Pavelski beat Frans Nielsen on a draw and the Sharks captain found a load of room to Halak's right as Justin Braun let a shot go from the point. The puck caromed off Pavelski and in.
And the lead stood into the intermission even through Mikhail Grabovski had a penalty shot attempt with 2:13 left in the first. Grabovski tried to wait Stalock out but couldn't lift the puck over the goaltender's right pad, leaving the Isles 0-for-2 on penalty shots five games this season.
Much as the Islanders did on Thursday after a lackluster first period against the Predators, they were much more resilient in the second. Johnny Boychuk let one fly off a Tavares faceoff win and the shot banked off the crossbar and in just 21 seconds into the second and the Isles were on the board.