Tidmarsh is a village and civil parish in the English county of Berkshire, on the A340 road between Pangbourne and Theale. It lies just north of the M4 motorway. It is south of Pangbourne, west of Reading and west of London.
The smaller village of Sulham is adjacent to Tidmarsh on the east side. The two villages share the combined parish council of Tidmarsh with Sulham. Further east, Sulham Woods separate the villages from Tilehurst, the western suburb of Reading.
This section of the A340 is thought to follow the Roman road from Silchester, alias Calleva Atrebatum (about south), either to Dorchester-on-Thames (about north) or a river-crossing at Pangbourne. It has, however, been artificially straightened in more recent times.
Tidmarsh has two historic buildings. The most conspicuous is the 13th century Greyhound Pub, now reopened following a serious fire in 2005. The other is the 12th Century church of St Laurence, Tidmarsh. The Norman doorway of the church is its most notable feature.
The River Pang flows north through the village on its way to join the River Thames at Pangbourne. The river flows through the Moor Copse Nature Reserve, which has recently (December 2006) been doubled in size, to about . The Tidmarsh and Sulham circular walk, of length about 2.5 miles, passes through the reserve and both villages.
The most famous people to live in Tidmarsh were the author Lytton Strachey (1880-1932) and the painter Dora Carrington (1893-1932). A recent housing development at the north end of the village has been named 'Strachey Close'.