William Kyriakos Jr. (born 3 September 1972) is a Brazilian former professional tennis player.

Kyriakos, who comes from São Paulo, represented Brazil at the 1991 Pan American Games in Havana.

He won a silver medal in the mixed doubles competition, partnering Cláudia Chabalgoity, as well as a gold medal in the team event.

The team of Kyriakos, Nelson Aerts and Marcelo Saliola defeated Puerto Rico in the final.

[1] On the professional tour, Kyriakos had a career best singles ranking of 266 in the world and made an ATP Tour main draw appearance as a qualifier at the Brasília Open, held once only in 1991.