The Marlborough Sounds is an extensive network of sea-drowned valleys at the north of the South Island of New Zealand, connected with Cook Strait. Islands in the sounds include Arapawa Island and D'Urville Island. The main port is Picton on the mainland. The ferry service between Picton and Wellington in the North Island travels through the sounds.

There are many small settlements and isolated houses only accessible by boat.

Marine farming is increasingly common, although the wake caused by fast catamaran vehicular ferry services to the North Island has been alleged to damage farms, and resulted in a dispute heard in the environment court. The fast ferries are now restricted to a lower speed in the sounds, reducing their time advantage over the conventional ferries.

The most notable shipwreck in the sounds is that of the Russian cruise liner Mikhail Lermontov, which sank in 1986 in Port Gore after striking rocks. One life was lost in the incident, which is documented on the website of the NZ Maritime Museum