John Thirtle (22 June 1777 – 30 September 1839) was an English watercolour artist. Born in Norwich, where he lived for most of his life, Thirtle was a leading member of the Norwich School of painters, whose paintings of Norwich are considered to be outstanding in the history of watercolour painting.
Thirtle was apprenticed to a London frame maker before returning home to Norwich. There he set up a successful frame-making business, whilst continuing to paint as well as working as a drawing-master, a printseller and a looking glass maker. In 1812 he married Elizabeth Miles, the sister-in-law of John Sell Cotman. He suffered from tuberculosis for much of his life, and his worsening health reduced his artistic output up to his death in 1839. He produced relatively little: his unpublished manuscript Treatise on Watercolour was probably for his own use, and he exhibited fewer than a hundred works of art. He was a member of the Norwich Society of Artists and for a time its vice-president, but in 1816 he was one of a number of artists who seceded from the Society to form a separate association, which dissolved after only three years.
The majority of Thirtle's watercolours are of Norwich and the surrounding Norfolk countryside, many being riverside scenes. His style, which was influenced by Thomas Girtin, Crome and (to a lesser extent) John Sell Cotman, was both technically accomplished and individual. His earlier landscapes were painted with a restricted range of buffs, blues and grey-browns, but he later developed a brilliancy of colour, producing works that included angular block forms. Unhappily, the quality of several of his watercolours has deteriorated, due to the fading of a particular indigo pigment that he used extensively and to great effect.