John Thirtle was an English watercolour artist and frame-maker. Born in Norwich, where he lived for most of his life, he was a leading member of the Norwich School of painters.
Much of Thirtle's life is undocumented. After working as an apprentice to a London frame-maker, he returned to Norwich to establish his own frame-making business. During his career he also worked as a drawing-master, a printseller and a looking glass maker. He produced frames for paintings by several members of the Norwich School, including John Crome and John Sell Cotman. Throughout his working life he continued to paint. In 1812 he married Elizabeth Miles, the sister of Cotman's wife Ann. Thirtle suffered from tuberculosis during the last two decades of his life, and his worsening health reduced his artistic output up to his death in 1839. His unpublished manuscript Treatise on Watercolour was probably for his own use, and he exhibited fewer than 100 paintings.