The news was announced by Rowling herself in a press release today. Per the statement, the new play is set to "explore the previously untold story of Harry’s early years as an orphan and outcast," when he was still living with the Dursley family in the London suburb of Surrey with no clue about his magical powers or the real reason behind his parents' death.
According to Badass Digest, the play is set to open in London's West End in two years, and will be produced by Rowling along with veteran producers Sonia Friedman and Colin Callender. Unfortunately, Rowling will not be writing the play—she'll instead be "collaborating" with a playwright who has yet to be announced.
No specific opening date has been set, yet.