The Battle of the Downs took place on 21 October 1639 (New Style), during the Eighty Years' War. A Spanish fleet, commanded by Admiral Antonio de Oquendo, was decisively defeated by a Dutch force under Lieutenant-Admiral Maarten Tromp. Victory ended Spanish efforts to re-assert naval control over the English Channel and confirmed Dutch dominance of the sea lanes, while it is also alleged to be the first major action to feature line of battle tactics.

The battle was initiated when Spanish chief minister Olivares sent a large convoy of troops and supplies for the Army of Flanders, escorted by some 50 warships. Since 1621, Spanish naval activity in the Channel had focused on avoiding direct conflict with the superior Dutch fleet, while attacking their merchant ships from bases in Dunkirk and Ostend. In a change from this policy, Oquendo was ordered to deliver the reinforcements but also bring the Dutch to battle; Olivares hoped victory would restore Spanish prestige and force the States General to negotiate peace terms.

The Spanish entered the Channel on 11 September and were intercepted by the Dutch in a series of actions between 16 and 18 September. Losses on both sides were minimal, but Oquendo took refuge in The Downs, an anchorage between the ports of Dover and Deal, where he was protected by English neutrality. Although blockaded here by the Dutch fleet, most of the reinforcements were transported to Dunkirk via small, fast frigates.

On 21 October, the Dutch entered the Downs and attacked the Spanish fleet with fireships. Unable to manoeuvre in the cramped waters and with the wind against them, the Spanish lost around ten ships captured or destroyed, while another twelve deliberately ran themselves ashore to avoid capture. Combined with the repulse of a similar-sized expedition against Dutch Brazil in January 1640, this marked the end of attempts to challenge Dutch maritime supremacy and an acceptance by the Spanish court that the war could not be won.