The Battle of Nechtansmere was fought between the Picts and Northumbrians on May 20, 685. It ended in a decisive Pictish victory and severely weakened Northumbria's power in northern Britain.

The Northumbrians had been gradually extending their territory to the north, having captured Edinburgh in 638. For the next thirty years they established political dominance over the Kingdoms of Strathclyde and Dalriada, as well as some of the Picts further to the north.

King Ecgfrith of Northumbria invaded lands held by the Picts in 685, apparently to stop them from raiding to the south. They met in battle on May 20 near Dunnichen ; the Picts pretended to retreat, drawing the Northumbrians into the swamp of Nechtansmere. The Pictish King Bridei III killed Ecgfrith and destroyed his army and enslaved many of the survivors. After the battle, Northumbria's influence never again extended past the Firth of Forth.

Little is known about the actual battle; it was briefly described by the Venerable Bede in the 8th century.