5★STAR Grand Prix 2021 in Korakuen ~ Day 1 was a professional wrestling event promoted by World Wonder Ring Stardom, held on August 13, 2021 at Korakuen Hall in Tokyo, Japan. It served as the sixth show for the company's annual round-robin singles tournament and included five matches from the field of 20 competitors, main evented by DDM's Giulia taking on STARS' newly-returned Koguma as part of the Red Stars block.