The Walcheren Campaign (Dutch pronunciation: [ˈʋɑlxərə(n)]) was an unsuccessful British expedition to the Kingdom of Holland in 1809 intended to open another front in the Austrian Empire's struggle with France during the War of the Fifth Coalition. John Pitt, 2nd Earl of Chatham, the commander of the expedition, was ordered to capture the towns of Flushing (Vlissingen) and Antwerp and thus enable British ships to safely traverse the Scheldt River.

A British expeditionary force of 39,000 troops, together with field artillery and two siege trains, crossed the North Sea and landed at Walcheren on 30 July. This was the largest British expedition of that year, larger than the army serving in the Peninsular War in the Iberian Peninsula. Nevertheless, it failed to achieve any of its goals. The campaign involved little fighting, but heavy losses from the sickness popularly dubbed "Walcheren Fever". Despite initial local successes against scattered French forces, Lord Chatham's slow and indecisive actions, combined with the arrival of French Marshal Bernadotte (the future Swedish king) at the front—who managed to concentrate up to 40,000 troops in Antwerp and then strengthen the beleaguered forts—stalled the British army and navy, which could no longer overcome the Franco-Dutch defenses. Lord Chatham did not dare to take other possible steps, for example, do not focus all his efforts on the forts and do try to move further.

On 11 September Marshal Bessières replaced Bernadotte as commander-in-chief and oversaw the disintegration of the British army until December. The reason for the replacement was Napoleon's distrust and disrelish of Bernadotte, whom he saw as vain since Bernadotte was overstating his capabilities without fear of consequences.

The Walcheren campaign is a typical example of a poorly thought-out and indecisive operation that cost Britain a lot of money and human lives, but did not produce any positive results. Although more than 4,000 British troops died during the expedition, only 106 were killed in action; the survivors withdrew on 9 December. Defending Franco-Dutch forces also suffered heavy casualties, including 4,000 men dead, wounded or captured.

The blockade of the Scheldt was declared as early as 14 May 1809, but no reconnaissance or sounding of the entrance fairways has been carried out before the campaign began.

In July 1809, the British decided to seal the estuary of the Scheldt to prevent the port of Antwerp being used as a base against them. The primary aim of the campaign was to destroy the French fleet thought to be in Flushing while providing a diversion for the hard-pressed Austrians. However, the Battle of Wagram had already occurred before the start of the campaign and the Austrians had effectively already lost the war.

John Pitt, 2nd Earl of Chatham commanded the army, and Sir Richard Strachan commanded the navy, the full expeditionary force of 37 ships, the greatest to have ever left England, leaving The Downs on 28 July. Commanders included Hugh Downman, Edward Codrington, Amelius Beauclerk, William Charles Fahie, George Cockburn and George Dundas.

As a first move, the British seized the swampy 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 "Walcheren fever", due to the symptoms present most likely a combination of malaria, typhus, typhoid and dysentery. Within a month of seizing the island, they had over 8,000 fever cases. The medical provisions for the expedition proved inadequate despite reports that an occupying French force had lost 80% of its numbers a few years earlier, also due to disease. Once it had been decided to garrison Walcheren Island in September 1809, Pitt was replaced by Lieutenant-general Eyre Coote who in October was replaced by Lieutenant-general George Don.