The naval Battle of the Downs took place on 31 October 1639 during the Eighty Years' War and was a decisive defeat of the Spanish, commanded by Admiral Antonio de Oquendo , by the United Provinces, commanded by Admiral Maarten Tromp.

In 1639 the Spanish prepared a force of 77 ships carrying 24,000 soldiers and sailors, planning to take control of the English Channel from the Dutch and land the army at Dunkirk. They were sighted by the Dutch in early October in the Channel. Tromp had only 13 ships, but he attacked with such ferocity and skill that the Spanish were driven to take refuge on the coast of England, off the South Downs between Dover and Deal, near an English squadron commanded by Admiral Pennington.

Here Tromp was joined by Witte de With's fleet of 17 ships and together they blockaded the Spanish and sent urgently to the Netherlands for reinforcement. By the end of October Tromp had 105 ships and 12 fire ships. On 31 October he dispatched de With to watch the English and prevent them from interfering, and attacked. The large, unmanoeuverable Spanish ships were powerless to resist, and all but seven were taken or destroyed, leaving 15,200 dead and 1,800 prisoner. Oquendo managed to escape in the fog with seven ships and reach Dunkirk.

The complete victory destroyed Spanish sea power. Spain was no longer able to contest the control of the seas, and the Dutch and English were quick to take advantage and began to seize Spanish colonial possessions.

However, for England the Battle of the Downs was a humililation: a flagrant violation of English neutrality within sight of the English coast, with England's navy powerless to intervene. Lingering resentment from this incident may have influenced the breakout of the First Anglo-Dutch War not far from the Downs at the Battle of Goodwin Sands in 1652.