The 31-year-old actress last appeared on the CW drama as con artist Poppy Lifton in 2009.
She will reprise her role in a forthcoming episode, with a source telling Entertainment Weekly that Poppy "shows up in a surprising context to tangle with a new target".
Feldman appeared in five episodes of Gossip Girl's second season between 2008 and 2009, with her character attempting to con Serena (Blake Lively) and her family out of their money.
Andrea Gabriel (Lost), Barry Watson (What About Brian) and French model Zoe Aggeliki will also appear in the show's final episodes, while Michelle Trachtenberg has confirmed that she will be back as devious socialite Georgina Sparks.
The sixth season of Gossip Girl - which will be just ten episodes long - premieres on October 8 in the US. In the UK, the series airs on ITV2.
> Leighton Meester: 'Gossip Girl ending is bittersweet'
Watch a clip from the Gossip Girl season five finale below: