Gary Siew Leong Koo^[2] (born 1968)^[1] is an Australian Anglican bishop who, since 20 December 2019, has served as an assistant bishop in the Anglican Diocese of Sydney, becoming the third Bishop of the Western Region.^[3][4]

Koo was born in 1968 in Sydney, as the son of two Malaysian migrants. He was educated at James Ruse Agricultural High School^[5] and then studied medicine at the University of Sydney.^[6] Koo was brought up as an atheist, but the death of his mother while he was training as a doctor at University caused him to reassess his views and he converted to Christianity.^[7]

After graduating from university, Koo worked as a doctor for a time.^[7] He then decided to study theology at Moore Theological College.^[3] and was ordained deacon in 2006 and priest (presbyter) in 2013.^[6]

Koo served as a pastor and church planter for the Chinese Christian Church Sydney. He later was appointed as a minister within the Anglican Diocese of Sydney at St Paul’s Carlingford and North Rocks, where he was eventually made rector, a position which he held for seven years prior to his appointment to the episcopate.^[6] He served as acting rector at Berala from 2013 to 2015.^[1]

In November 2019, Koo was appointed as Bishop of the Western Region in the Sydney Diocese, a role which involves overseeing congregations in the Blue Mountains, Hills District, Hawkesbury, and Parramatta areas.^[8] He replaced Bishop Ivan Lee, who was the first Anglican bishop in Australia to have a Chinese ethnic background and who moved at that time into a new episcopal role strengthening church growth.^[9][3] Koo was consecrated bishop in St John's Cathedral, Parramatta by Archbishop Glenn Davies on 20 December 2019.^[3]

As assistant bishop, Koo has chaired the Diocese's COVID-19 taskforce throughout 2020.^[10]

Koo is married to Pearl and has two children.^[6]