The Four Days Battle was a naval battle of the Second Anglo-Dutch War. Fought from June 11 to June 14, 1666 off the Flemish and English coast, it remains one of the longest naval engagements in history.

In June 1665 the British had soundly defeated the Dutch in the Battle of Lowestoft, but failed to take advantage of it. The Dutch Spice Fleet, loaded with fabulous riches, managed to return home safely. The Dutch navy was enormously expanded through the largest building programme in its history. In August 1665 already the British fleet was again challenged, though no large battles resulted. In 1666 the British became anxious to destroy the Dutch navy completely before it could grow too strong and were desperate to end the activity of Dutch raiders as a collapse of British trade threatened.

On learning that the French fleet intended to join the Dutch at Dunkirk, the English decided to prevent this by splitting their fleet. Their main force would try to destroy the Dutch fleet first, while a squadron under Prince Rupert was sent to block the Strait of Dover against the French - who didn't show up.

The English fleet of 56 ships commanded by George Monck, 1st Duke of Albemarle, was outnumbered by the Dutch fleet of 85 commanded by Admiral Michiel de Ruyter. The battle ended with a successful English disengagement after both fleets had expended most of their ammunition.

The Dutch inflicted significant damage on the English fleet. The English had gambled that the crews of the many new Dutch ships of the line would not have been fully trained yet but were deceived in their hopes: they lost seventeen ships, thousands of men and two admirals, Sir Christopher Mings and Sir William Berkeley . HMS Sovereign of the Seas was knocked out of the battle after she lost so many sailors that she could no longer move or fire her cannon.

On the third day the English retreated to the west. The grounded Prince Royal surrendered with admiral George Ayscue and was burned. In the evening Rupert, having already on the first day been ordered to join Monck, at last appeared with twenty ships.

Thus enforced Monck again attacked in line on the fourth day. But the Dutch had the weather gauge: the English attack faltered, their line was broken, the Dutch started to surround them. When the English regrouped and successfully broke free to the west escaping in a dense fog, only some stragglers being boarded by the Dutch, De Ruyter decided to break off the pursuit. His own fleet was heavily damaged too and he perhaps feared to run out of gunpowder while Rupert's squadron, despite now also being severely damaged (Rupert's flagship the Royal James was demasted), still had a sufficient supply; his log book only speaks of a fear for the English shoals. The deeply religious De Ruyter interpreted the sudden unseasonly fog bank as a sign of God, showing "that He merely wanted the enemy humbled for his pride but preserved from utter destruction".

It was the biggest sea battle of the Second Anglo-Dutch War. However, the outcome is often described as inconclusive, because both sides initially claimed victory. Immediately after the battle some English captains claimed De Ruyter had retreated first, then normally seen as an acknowledgement of the superiority of the enemy fleet. Though the Dutch fleet was eventually forced to end the pursuit, they had managed to cripple the English fleet. The contemporaneous Dutch view on this matter is expressed in a famous epigram by the poet Constantijn Huygens:

Two fight - and for their lives.

The one that caused the row

is beaten - but survives.

And boasts: "I've won it now!"

Two months later the recuperated English fleet challenged the Dutch fleet again, now much more successfully at North Foreland in the St. James's Day Battle. Though this proved to be a victory, the Dutch fleet again wasn't destroyed and the enormous costs of repair after the previous battle had depleted the British treasury, so the Four Days Battle is usually seen as a decisive strategic victory for the Dutch.