Penmanshiel Tunnel is a now-disused railway tunnel near Grantshouse, Berwickshire, in the Scottish Borders region of Scotland. It was formerly part of the East Coast Main Line between Berwick-upon-Tweed and Dunbar.

The tunnel was constructed during 1845–46 by the contractors Ross and Mitchell, to a design by John Miller, who was the Engineer to the North British Railway. Upon completion, the tunnel was inspected by the Inspector-General of Railways, Major-General Charles Pasley, on behalf of the Board of Trade.The tunnel consisted of a single bore, 244 metres (267 yd) long, containing two running lines.

During its 134-year existence, the tunnel was the location of two incidents investigated by HM Railway Inspectorate. The first was in 1949, when a serious fire destroyed two carriages of a south-bound express from Edinburgh. Seven passengers were injured, but there were no deaths.The second incident occurred on 17 March 1979 when, during improvement works, a length of the tunnel collapsed. Two workmen were killed, and 13 others managed to escape. Later it was determined that the ground was not stable enough to excavate and rebuild the tunnel, so it was sealed up and a new alignment was made for the railway, in a cutting to the west of the hill.The tunnel was also affected by the August 1948 floods. The damage caused by these floods led to the abandonment of much of the railway network in the south east of Scotland.