James Merriman Archer Thomson (5 February 1863 – 31 July 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).
From 1896 until his death (by suicide, he drank carbolic acid) in 1912 he was Headmaster of Llandudno County School.