James Merriman Archer Thomson (1863-1912) was a British rock-climber and mountaineer.
Thomson climbed extensively in North Wales with others such as Oscar Eckenstein. He was a believer in leading on sight, and was not keen on O. G. Jones's practice of surveying a route with a top-rope. He extensively explored the cliffs of Lliwedd, and in 1909 with A. W. Andrews, produced the first official climbing guide to the cliff (published by the then-recently-founded Climbers' Club). He authored the second, to the Ogwen District, the following year.
From 1896 until his death (by suicide, he drank Carbolic acid) in 1912 he was Headmaster of Llandudno County School.