Benoist, 26, will play Superman's cousin, Kara Zor-El, who escapes the planet Krypton and is taken in by an Earth family.
Benoist's suit was designed by Oscar-winner Colleen Atwood, who has worked on Into the Woods, Snow White and the Huntsman and CW superhero series Arrow. "In designing Supergirl, I wanted to embrace the past but more importantly, thrust her into the street-style action hero of today," Atwood said in a statement.
Benoist has previously starred in TV's Glee and the Oscar-nominated film Whiplash.
"I wish there was a superhero emoji," she tweeted Jan. 22 when her casting was announced. "But seriously, sans emoji... I AM SO EXCITED !!!!!!!!!!!!!!!!!! #Supergirl."