The Battle of Lowestoft, 13 June 1665 was a naval battle of the Second Anglo-Dutch War. It remains the worst naval defeat in Dutch history.

A fleet of more than a hundred ships of the United Provinces commanded by Lieutenant-Admiral Jacob van Wassenaer Obdam attacked an English fleet of equal size commanded by James Stuart, Duke of York forty miles east of the port of Lowestoft in Suffolk, England. The Dutch were desperate to prevent a second English blockade of their ports, after the first was broken off by the English for lack of supplies. The leading Dutch politician, Johan de Witt, ordered Van Wassenaer to attack the English aggressively during a period of stable eastern winds which would have given the Dutch the weather gauge. Van Wassenaer however, feeling that his fleet was still too inferior in training and fire power to really challenge the British in full battle, postponed the fight till the wind turned in order to seek a minor confrontation in a defensive leeward position from which he could disengage quickly and return without openly disobeying orders. His attitude would cost him a third of his fleet and his life.

The action was fierce and lasted all day. Confusion during a delicate fleet manoeuvre made the seven Dutch squadrons lose all coherence; they were then trapped by the British rear, blocking the intended return to the Dutch coast. The Dutch flagship Eendracht duelling the Royal Charles - James was nearly killed by a Dutch chain-ball decapitating several of his courtiers, the head of one of them striking him down - exploded, killing van Obdam. Lieutenant-Admirals Egbert Bartholomeusz Kortenaer and Auke Stellingwerf had already been killed at the beginning in separate actions. By evening the Dutch were routed, having lost about thirty ships, but Vice-Admiral Cornelis Tromp and Lieutenant-Admiral Johan Evertsen both having taken over command (showing the utter confusion on the Dutch side), made possible an escape and covered the flight, thus preventing complete catastrophe. The British lost only two ships, among which the captured Charity. Seventeen or sixteen Dutch ships were sunk by the British, nine were captured and seven more were burnt when they got entangled and collided with a Dutch fire ship.

Notable English captains present at the battle included Captain of the Fleet William Penn in the Royal Charles, Edward Montagu, 1st Earl of Sandwich commanding the rearguard, Prince Rupert commanding the centre and George Ayscue.

The outcome of the battle was partially caused by an inequality in fire-power, but the Dutch had already embarked on an ambitious expansion programme, building many heavier ships. The English failed to take advantage of their victory. The fleets, now much more equal in quality, met again at the Four Days Battle in June 1666.