The Walcheren expedition (July 30 - December 10, 1809), a British military operation during the Napoleonic Wars, formed the last in a series of operations in Flanders (present-day Belgium) in 1809. In the largest British expedition of that year, around 40,000 soldiers crossed the North Sea.

The expedition aimed to attack the French-controlled naval base at Antwerp and to provide a diversion for the hard-pressed Austrians (unfortunately the Battle of Wagram occurred before the expedition got under way). As a first move, the British seized the swampy, fever-ridden island of Walcheren at the mouth of river Scheldt as well as South Beveland island (both in the present-day Netherlands). The British troops soon began to suffer from malaria, within a month of seizing the island they had over 8,000 fever cases. The medical provisions for the expedition proved completely inadequate.

The operation was commanded by John Pitt, Lord Chatham (Army) and Sir Richard Strachan (Navy). Chatham had a reputation as an extremely cautious commander, and gave the operation a dangerously slow pace. While the British troops were diverted into the capture of Flushing (August 15) and surrounding towns, the enemy heavily reinforced Antwerp. With the main objective out of reach, the expedition was called off in early September. Around 12,000 troops stayed on Walcheren, by October only 5,500 remained fit for duty.

In all the British government had wasted almost £8 million on the expedition, 4,067 men had died (only 106 in combat). Almost 12,000 were still ill in February 1810 and many others remained permanently weakened.