St Agnes is a suburb of Adelaide, South Australia. It is around 15 km northeast of the city centre. It is located in the City of Tea Tree Gully.
Facilities and amenities on the suburb include Ardtornish Primary School, founded in 1881, and the Steventon Post Office that opened around January 1859, to be renamed Tea Tree Gully in 1872, and Teatree Gully in 1925. Tea Tree Gully changed again in 1966 and finally became St Agnes in 1969.