The 19-year-old actor took to Twitter to announce the role saying she will be playing Ethel Muggs, a character traditionally similar to Barb only more boy-obsessed.
She also told her followers how proud she was that someone one of her size and shape had made it in Hollywood.
Purser will join Kiwi actor KJ Apa, who is leading the cast as its main character Archie Andrews.
The show has been described as a "subversive take" on the comics, with Archie, Betty, Veronica and the rest of the gang exploring "the darkness and weirdness bubbling beneath Riverdale's wholesome facade".
Apa's Archie is reportedly an "intense, conflicted teen", juggling his passion for music with the interest of several girls, one of whom may be Purser.
Entertainment Weekly reports Purser will have a recurring role in the first few episodes in which she'll be involved in a romance with someone who isn't Jughead - her love interest from the original Archie.
The pilot for Riverdale is expected to air in the US in January or February 2017.
