The US actor, 48, will reprise her role of vampire-battling high school student Buffy Summers in the new series on Hulu, and in a post on social media told Star Wars: Skeleton Crew star Ryan Kiera Armstrong she was excited for her to be "my partner on this journey".
In the Instagram post, Gellar wrote: "I want to introduce you to @ryankarmstrong. From the moment I saw Ryan's audition, I knew there was only one girl that I wanted by my side.
"To have that kind of emotional intelligence , and talent, at such a young age is truly a gift. The bonus is that her smile lights up even the darkest room. Welcome to #NewSunnydale."
In the video, Gellar asks Armstrong: "How do you feel about helping me save the world? Do you want to be my chosen one? Will you stand by my side and save the world?"
In a post to her social media, Armstrong wrote: "I'm so beyond thankful and excited to share that I am going to be apart of the BTVS universe!!!
In the comment section, David Boreanaz, who played vampire Angel in the TV series, said: "Congratulations and Welcome! Enjoy the ride."
Armstrong, 15, has starred in TV series including American Horror Story and Anne With An E.
Gellar in February said she was "on the path" to reviving Buffy the Vampire Slayer alongside a team of "four unbelievably talented women".
The series, created by writer and director Joss Whedon, saw Gellar play Buffy Summers, one of a long line of young women chosen by fate to battle supernatural forces in the fictional US town of Sunnydale.
The program also starred Alyson Hannigan as Willow Rosenberg, James Marsters as Spike and British star Anthony Head as Rupert Giles.
The show's success led to a spin-off series for vampire Angel, played by Boreanaz.
Gellar has also starred in 1990s movies Cruel Intentions and I Know What You Did Last Summer and plays the boss of fictional serial killer Dexter Morgan in a new prequel series.
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