Constantin Frantzen (born 16 March 1998) is a German tennis player who specializes in doubles.

[1] He has won seven ATP Challenger doubles titles, all with partner Hendrik Jebens.

[2] 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 Zieliński.

[3] Frantzen reached his second ATP Tour final at the 2024 Generali Open Kitzbühel also alongside Jebens but lost to Alexander Erler and Andreas Mies.