Dartmouth is set on the banks of the estuary of the River Dart, which is a long narrow tidal ria that runs inland as far as Totnes. It lies within the South Devon Area of Outstanding Natural Beauty.

Dartmouth was of strategic importance as a deep-water port for sailing vessels. The port was used as the sailing point for the crusades of 1147 and 1190, and a creek close to Dartmouth Castle is supposed by some to be named for the vast fleets which assembled there (Warfleet Creek). It was a home of the English navy since the reign of Edward III and was twice surprised and sacked during the Hundred Years' War, after which the mouth of the estuary was closed every night with a great chain. The narrow mouth of the Dart is protected by two fortified castles, Dartmouth Castle and Kingswear Castle.

The town is dominated by the Royal Navy Officer Training college (Britannia Royal Naval College) and all officers of the Royal Navy, as well as many foreign navies, are trained there.

The Port of Dartmouth Royal Regatta takes place annually over three days at the end of August.

Thomas Newcomen, the inventor of the steam pumping engine was born in Dartmouth in 1663. The location of his house in Lower Street is marked with a plaque, although the building itself was demolished (and reconstructed on Ridge Hill) in the nineteenth century to make way for a new road which was named after Newcomen.