Constantin Frantzen (born 16 March 1998) is a German professional tennis player who specializes in doubles. He has a career-high ATP doubles ranking of world No. 34 achieved on 16 March 2026.
Frantzen has won one ATP Tour doubles title at the 2025 Chengdu Open, with Robin Haase.
He played college tennis at Baylor University.
Frantzen made his ATP Tour doubles main draw debut at the Swiss Open Gstaad alongside Hendrik Jebens.
He reached his first ATP Tour final at the Moselle Open with partner Jebens, but lost to Hugo Nys and Jan Zielià Âski. Frantzen reached his second ATP Tour final at the 2024 Generali Open Kitzbühel also alongside Jebens but lost to Alexander Erler and Andreas Mies. The pair also made it to the 2024 Hangzhou Open final, losing to Jeevan Nedunchezhiyan and Vijay Sundar Prashanth in the final.
In September, Frantzen won his first doubles title on the ATP Tour at the Chengdu Open, playing alongside Robin Haase.
Current through the 2026 Miami Open.