The 23-year-old Australian actor will appear as a series regular, playing Wally West, the iconic character from the DC Comics universe who goes on to become the speedster known as Kid Flash, reported Entertainment Weekly.
"Just like when we met Grant for the first time, we instantly knew Keiynan embodied all the heart and courage of a hero. We are so excited to be bringing this much-beloved character onto the show," executive producer Andrew Kreisberg said.
In comics, Wally is the nephew of Iris West (played in The CW series by Candice Patton), the love interest and eventual wife of Barry Allen (Grant Gustin).
"The Flash's" major antagonist on season two would be Zoom.
The show is expected to return this fall.