Katie Jill O'Brien (born 2 May 1986) is a British former professional tennis player from Beverley, Yorkshire. She was briefly the British No. 1 tennis player, and reached her career-high singles ranking of world No. 84 on 1 February 2010. She won four singles and two doubles titles on the ITF Circuit. In 2007, she reached the second round of her home Grand Slam, Wimbledon, by beating Sandra Klösel in round one. She lost to the No. 31 seed, Michaëlla Krajicek, in the second round. In 2010, she replicated this achievement by beating Patricia Mayr to reach the second round of the Australian Open where she fell to eighth seeded Jelena Janković.