John Michael Hammersley (21 March 1920 – 2 May 2004) was a British mathematician best known for his foundational work in the theory of self-avoiding walks and percolation theory. He was born in Helensburgh in Dunbartonshire, and educated at Sedbergh School. He started reading mathematics at Emmanuel College, Cambridge but was called up to join the Royal Artillery in 1941. During his time in the army he worked on ballistics.
He graduated in mathematics in 1948. He held a number of positions, both in and outside academia. His book Monte Carlo Methods with David Handscomb was published in 1964.
He was an advocate of problem solving, and an opponent of abstraction in mathematics, taking part in the New math debate.
He was a Fellow (later Professorial Fellow) of Trinity College, Oxford from 1961, Reader in Mathematical Statistics at Oxford University from 1969, and elected Fellow of the Royal Society in 1976.