Willow Smith is fulfilling a lifelong dream.

The 17-year-old singer-actress will guest star on the series finale of Cartoon Network's Adventure Time, airing Sept. 3, ET has exclusively learned.

Smith, who is a big fan of the network's longest-running animated series, will voice the character of Beth -- and only ET has the exclusive first look of her hard at work in the recording studio. (Smith's character is the taller of the two, on the right, in the top photo below).

Cartoon Network

Fun fact: "Marceline," a song off Smith's 2015 debut album, Ardipithecus, is based off the Adventure Time character, Marceline the Vampire Queen.

Smith will also be featured on the official Adventure Time finale episode soundtrack, covering the show's main title theme.

Netflix

Featuring over 20 tracks from the series' swan song, the forthcoming soundtrack also includes original music by composer Tim Kiefer, a new song by Rebecca Sugar and the full-length version of Ashley Eriksson's "Island Song (Come Along With Me)." It will be available for pre-order starting Saturday, Sept. 1 and available on streaming services Tuesday, Sept. 4.

Additionally, a special edition vinyl LP of the soundtrack released by Mondo will be out this fall and is available for pre-order.

Adventure Time follows the adventures of Finn the Human and his best friend/adoptive brother, Jake the Dog, who has magical powers to change shape and size at will. The series premiered the pilot in January 2007, before embarking on a 10-season run starting in April 2010, airing nearly 300 total episodes.

Adventure Time airs its extended series finale Monday, Sept. 3 at 6 p.m. ET/PT on Cartoon Network. The Adventure Time: The Final Seasons DVD will be released on Tuesday, Sept. 4, alongside the soundtrack release.

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