The Scotland international keeper was in the final year of an initial two year deal at Elland Road which he signed when he arrived from Chelsea in July last year.
Sullivan was a free-transfer signing from the Londoners and has so far made 53 appearances for the Whites, being player of the year last season.
A thigh injury has restricted him to just three outings this season. Kevin Blackwell says keeping the 35-year-old is another boost for the squad.
"I am delighted Neil is prepared to commit his future to the club," said Blackwell on the official Leeds website.
"He can see the club is moving in the right direction now and he wants to be part of the future here, so it is more good news for us.
"To take the club where we want to go, we need to be able to sign reliable, good players and Neil Sullivan is certainly one of those."