Dame Gillian Constance Weir DBE FRCM (born 17 January 1941) is a New Zealand-British organist.
Weir was born in Martinborough, New Zealand in 1941. When she was 19, she was a co-winner of the Auckland Star Piano Competition, playing Mozart. A year later she won a scholarship of the Associated Board of the Royal Schools of Music in London. There, she studied with the concert pianist Cyril Smith and the renowned organist Ralph Downes, and in her second year (1964) won the prestigious St. Albans International Organ Competition.
Her performance in 1964 of a work by Olivier Messiaen occurred at a time when his music was little-known outside France and she became particularly associated with this composer; she has several times performed his complete works in series. Her recording for Collins Classics (new re-release for Priory Records on 2002) was hailed as "one of the major recording triumphs of the century" in In Tune Magazine. Her distinguished position as a Messiaen interpreter has been reinforced by her CD release of his complete organ works to great acclaim as well as by her contribution to Faber's The Messiaen Companion and other publications. At Messiaen's request, she gave the first UK performance in January 1973 of the Méditations sur le Mystère de la Sainte Trinité at the Royal Festival Hall from the composer's manuscript, given to her after he gave the world premiere in Washington D.C.