Joannes Henricus "Joop" Knottenbelt internationally known as Josef Knottenbelt (1910 – 1998) was a Dutch tennis player. He was a member of the Netherlands Davis Cup team with among others Henk Timmer.

In 1927 he won the Dutch youth (under 18) championships.^[3] In 1931, Knottenbelt reached the second round of the men's singles at Wimbledon, losing to Eberhard Nourney of Germany, 8–10 6–2, 0–6 9–11.^[4] In the 1932 Wimbledon mixed doubles, he and Madzy Rollin Couquerque lost in the fourth round to Ellsworth Vines and Helen Wills-Moody.^[5][6]

In the quarterfinals of the 1934 Davis Cup, he lost to Swedes Curt Östberg (1–6, 1–6, 4–6) and Kalle Schröder (1–6, 5–7, 0–6) although the Netherlands won the round.^[7]

Knottenbelt was born in the Dutch East Indies but grew up in Bussum. His younger brother Anthony Knottenbelt was also a promising tennis player, but died at the age of 19 after an accident.^[8] In September 1937 Knottenbelt left again for the Dutch East Indies.^[9]