Brighton were knocked off the top of the Championship after Leicester ended their unbeaten start to the season.
The Foxes dominated the first half with Andy King and Sol Bamba being denied by Seagulls keeper Casper Ankergren.
And just 45 seconds after the break Yuki Abe powered in a header from Darius Vassell's left-wing cross.
Brighton's Craig Mackail-Smith volleyed over from three yards but Marcos Painter was sent off late on to all but end their hopes of salvaging a point.
Leicester were good value for their win though with pacy duo Vassell and Jermaine Beckford causing havoc up front.
And as Brighton pushed for an equaliser, they left gaps at the back which Beckford almost exploited. He broke and found Gelson Fernandes but the Swiss midfielder's shot cleared the crossbar.
Ryan Harley had two goalbound efforts blocked by Lee Peltier and Matt Mills as the visitors continued to venture forward.
But their adventure was checked when Painter was dismissed after picking up a second yellow card for a blatant handball with 10 minutes of the match remaining.
Middlesbrough, who won 1-0 at Crystal Palace, replace Brighton at the top of the table.