The Church of St Martin, Cwmyoy, Monmouthshire, Wales, was begun in the 12th century, although most of the current structure dates from the 13th century. The church is most notable for its extreme tilt, caused as a result of a landslide. This has led to many attempts to strengthen the church and prevent its collapse, through the use of massive tie beams and buttresses. The church is in the Gothic style, and has a chancel, nave, south porch, and western tower. A round-headed north window dates to the 12th century, and the nave roof has been dated to the late 13th or early 14th century. St Martin's is a Grade I listed building and an active parish church. It is dedicated to St Martin of Tours.
The underlying geology is of Old Red Sandstone overlying marl, and processes of slippage and subsidence over the centuries have caused the tower and chancel to move in opposite directions, twisting the entire structure. The initial landslips must have occurred soon after the original construction as the medieval fabric of the church shows evidence of repairs and rebuilding in attempts to address the instability of the site.
The tower is medieval but cannot be dated with certainty. A Victorian restoration by J. James Spencer in 1887 added buttresses to the nave and chancel and flying buttresses to the tower. These efforts sought to stabilise the church rather than to "set (it) straight". Further renovations took place in 1991. St Martin's is known as "the crooked church" due to the extreme nature of its non-alignment and has been called "the most crooked in Britain".