Thomas Collier RI was an English landscape painter.
Collier was born in Glossop in Derbyshire, the son of Martha Siddall and Thomas Collier, who was a prosperous grocer and tea dealer. He received tuition at the Manchester School of Art and, inspired by David Cox's example, lived at Betws-y-Coed in northern Wales between 1864 and 1869. He moved to London in about 1870 on being elected to the New Water Colour Society.
Collier was a close friend of Charles Stuart Millard, the Canadian-born painter.
Although not a prolific worker or exhibitor, he is regarded as one of the finest of English landscape watercolourists. He was made a Chevalier of the Legion of Honour in recognition of a painting sent to the Paris International Exhibition of 1878. Collier was industrious, retiring and often in poor health, yet financially independent, able to work without pandering to popular taste and to travel at will into the Suffolk countryside. In 1879 he arranged for the construction of a large house and studio at 9 Hampstead Hill Gardens in Hampstead where he spent his days painting and entertaining artist friends.thumb|right|9 Hampstead Hill Gardens, Collier's Hampstead residence, in 2021