frontier between Tibet and Assam in British India, negotiated between Tibet and Great Britain at the end of the Simla Conference (October 1913–July 1914) and named after the chief British negotiator, Sir Henry McMahon. It runs from the eastern border of Bhutan along the crest of the Himalayas until it reaches the great bend in the Brahmaputra River where that river emerges from its Tibetan course into the Assam Valley.

Delegates of the Chinese republican government also attended the Simla Conference, but they refused to sign the principal agreement on the status and boundaries of Tibet on the ground that Tibet was subordinate to China and had not the power to make treaties. The Chinese maintained this position until the frontier controversy with independent India led to the Sino-Indian hostilities of October–November 1962. In that conflict the Chinese forces occupied Indian territory south of the McMahon Line but subsequently withdrew after a ceasefire had been achieved.