St Mary's Church is the redundant Church of England parish church of the deserted medieval village of Fleet Marston, Buckinghamshire, England. It is recorded in the National Heritage List for England as a designated Grade II* listed building, and is under the care of the Churches Conservation Trust. The church stands in a field to the northeast of the A41 road, some 2 miles (3 km) northwest of Aylesbury. John Wesley preached his first sermon in the church soon after he was ordained deacon in September 1725.