Catherine Cartan "CiCi" Bellis (born April 8, 1999) is an American former tennis player. In early 2018, she was the second youngest player in the top 100 of the WTA rankings. Bellis has a career-high of 35, which she achieved in August 2017. She is known for winning a match at the 2014 US Open as a 15 year old against a top-20 opponent, making her the youngest match-winner at the US Open since 1996. She announced her retirement on Instagram on January 19, 2022.
Her biggest title came at the 2016 Hawaii Tennis Open, a WTA 125 event. She also won seven singles titles and two doubles titles on the ITF Circuit. Bellis had an accomplished junior season the same year she played in her first US Open, finishing the 2014 season as the ITF Junior World Champion for holding the year-end world No. 1 junior ranking. She also won the USTA National Junior Championship that year at 15 to become the youngest winner of the event since Lindsay Davenport in 1991.