Sea Mills is a suburb of Bristol, England, 3.5 miles (6 km) north-west of the city centre, between the former villages of Shirehampton, Westbury-on-Trym and Stoke Bishop, by the mouth of the River Trym where it joins the River Avon.

Since 2015, central and south-eastern Sea Mills have been in Stoke Bishop ward for elections to Bristol City Council and north-western Sea Mills in Avonmouth and Lawrence Weston ward, with a ward boundary along Sylvan Way (A4162). Sea Mills was previously in Kingsweston ward, with a ward boundary along the Trym.

Sea Mills derives its name from a watermill just above the tidal limit of the River Trym, recorded first in 1411 as Semmille and in 1484 as Cemille. This probably meant that its grinding capacity was limited to one packhorse-load of grain (a seam) but was later misinterpreted to mean 'mill by the sea'. The name was subsequently extended to an adjacent farm on the north side of the Trym, Seamill Farm, and to one of the earliest wet docks in England, Seamill Dock − where dock gates retained water at the high-tide level.

Constructed on 12 acres (5 hectares) of land where the Trym joins the Avon, leased from the King's Weston Estate in 1712, the dock was intended "to provide a repair and ‘laying up’ facility for ships docking at the congested and vastly overcrowded quays further up [the Avon] in Bristol". But poor land transport links with Bristol doomed the enterprise. The dock and associated warehouses were abandoned in the 1760s. It was described and partially surveyed by the author and inventor, George William Manby, in 1802. The ruined dock walls survive, and pleasure craft were moored in the much silted-up harbour until recently.

Sea Mill had ceased to function before 1800, but two watermills further up the Trym, near the Sea Mills boundary, remained in use until the 20th century: Clack Mill, below what is now the bend on Coombe Bridge Avenue, and Coombe Mill, beyond the Blaise Castle Estate car park in Coombe Dingle. Both had been demolished by the 1950s and their mill leats and a mill pond obliterated.

By the time the first Ordnance Survey map of this part of Gloucestershire was published, in 1830, the present name Sea Mills had become established for the farm, dock area and an early 18th century tavern on Sea Mills Lane opposite what is now Sea Mills Depot. The tavern was turned into a farmhouse soon afterwards and renamed The Hermitage. It was demolished in the 1930s, before Trym Cross Road was constructed, and Sea Mills Lane and the course of the Trym were realigned.

When the Bristol Port and Pier Railway Company standard gauge line (so named because Avonmouth as a port did not then exist) was opened beside the Avon in 1865, from Hotwells to a new deep water pier at Avonmouth, the station built on the south side of the Trym to serve the mansions and villas of the wealthy districts of Stoke Bishop and Sneyd Park was therefore called Sea Mills. The line was single track, standard gauge and, as built, unconnected to any other railway line. Of course, most other local railways were to Brunel's broad gauge. On opening it had twenty four-wheeled passenger coaches and six goods wagons so the emphasis was clearly on passengers and their time saving by catching steamers at the Avonmouth pier rather than in the centre of Bristol. On Saturday 3rd June 1865 the pier was opened when the S. S. Apollo landed passengers from Cork saving them at least four hours on a journey to London. Passenger services with Bristol’s principal railway station at Temple Meads were established in 1886, after the construction of a mile-long tunnel under the Downs from Clifton Down railway station to the Avon Gorge, and still operate. Whereas the original railway line between Hotwells and the connection with the Clifton Extension Railway at Sneyd Park Junction was closed in 1922 to make way for the Portway.

There was an ancient folk memory of a Roman port at the mouth of the Trym, and much Roman material was unearthed when Seamill Dock was constructed. All finds then and later have been on the south side of the river. In the 1820s it was proposed and generally accepted that this was the site of the port of Abona (Avon), linking Silchester and Bath with Venta Silurum (Caerwent) in Wales, on Route 14 of the 3rd-century Antonine Itinerary’s Britannia section. There was then no other port on the Avon or other town in the Bristol area. Piecemeal archaeological excavations have since found evidence of the street pattern, buildings within the small Roman town and cemeteries outside it.