
St Martin's is the oldest surviving church in Dorset and represents the most complete example of a Saxon church in the county. Its Saxon origin can be seen in the tall, narrow nave and chancel, late Saxon wall-arcading in the North West Isle and traces of a Saxon door. The building has been adapted and enlarged but the nave and a tiny window in the north side of the chancel are original to the church's construction in around 1030.

Fragmented 12th-century frescoes adorn the chancel, depicting events from the life of St Martin, whilst above the chancel arch is a Royal Arms of Queen Anne dated 1713, flanked by black letter scriptural texts of about 1600 which overlay earlier scripts.
A number of red stars have been painted on one of the walls, supposedly one for every person who died in the town during the great plague in the 17th century.
In the aisle is Eric Kennington's carved efficy of T E Lawrence in Arabian dress.