The winger has already earned Palace £15million after agreeing a switch to Manchester United in January, which will be completed in the summer. And a well-taken double means he could now leave the Eagles a parting gift of a place in the Barclays Premier League next season and the millions of pounds that go with it.
A tense derby encounter was going the same way as the goalless first leg until Zaha broke the deadlock in the 69th minute, just moments after Brighton substitute Ashley Barnes had twice come agonisingly close to giving the hosts the lead. And the 20-year-old England international killed off the Seagulls with a fabulous turn and shot with two minutes remaining.
It means Ian Holloway's Palace, who only secured their place in the top six on the final day of the season following a dismal run of form, will play Gianfranco Zola's Watford in the final in a fortnight.
It was a different story in the first half for Zaha, who struggled to make an impact and became frustrated at some of the close attention he was receiving from Brighton's defenders. Instead the Seagulls bossed the early stages with Julian Speroni having to keep out Will Buckley's drive before David Lopez's shot was deflected narrowly over.
Former England defender Matt Upson headed Andrea Orlandi's corner wide, Dean Hammond was also off target with a fierce, low drive and Orlandi's volley was straight at Speroni. But Palace were a different proposition after the break and Jonny Williams missed a gilt-edged chance to put them ahead when he was sent clean through by Aaron Wilbraham.
The young Wales midfielder's low shot beat the dive of Seagulls goalkeeper Tomasz Kuszczak but rolled just wide of the far post.
Brighton striker Barnes twice came even closer to breaking the deadlock just a minute after climbing off the bench. He latched onto Leonardo Ulloa's flick-on, only for Speroni to tip his shot onto the crossbar and from the resulting corner saw his header cleared off the line by Dean Moxey.
The fine margins which decide these play-off encounters were brought home to Brighton moments later. Palace sub Yannick Bolasie got the better of Inigo Calderon on the left and swung in a cross which Zaha, who had got in front of Wayne Bridge, met with a firm diving header which crashed into the net.
And as Brighton pushed for an equaliser they left a hole at the back which Kagisho Dikgacoi exposed by playing in Zaha, who turned Gordon Greer before thumping his shot into the roof of the net in front of the jubilant travelling fans.