The Axis capture of Tobruk, also known as the Fall of Tobruk and the Second Battle of Tobruk (17–21 June 1942) was part of the Western Desert campaign in Libya during the Second World War. The battle was fought by the Panzerarmee Afrika (Armata Corazzata Africa in Italian), a German–Italian military force in North Africa which included the Afrika Korps (Generalleutnant Erwin Rommel), against the British Eighth Army (General Neil Ritchie) which comprised contingents from Britain, India, South Africa and other Allied nations.

Axis forces had conducted the Siege of Tobruk for eight months in 1941 before its defenders, who had become an emblem of resistance, were relieved in December. Claude Auchinleck, the commander-in-chief Middle East Command, had decided not to defend Tobruk for a second time, due to the cost of bringing supplies in by sea; its minefields and barbed wire had been stripped for use in the Gazala Line to the west. By mid-1942 the Desert Air Force had been forced to move to airfields in Egypt, taking most of them beyond the range of Tobruk. About a third of all garrison personnel were non-combatant or support troops and many of the fighting troops were inexperienced. Lieutenant-General William Gott, the commander of XIII Corps, was withdrawn from Tobruk and on 15 June 1942, five days before the Axis attack. The new commander of the 2nd South African Division, Major-General Hendrik Klopper, was given command of the garrison. An immense stock of supplies had been accumulated around the port for Operation Acrobat but the Axis had forestalled this with Operation Venice (Unternehmen Venezia) and the Battle of Gazala began on 26 May 1942.

The Eighth Army was defeated in the Battle of Gazala and was driven eastwards toward the Egyptian border, leaving Tobruk isolated. The Prime Minister, Winston Churchill, placed great store on the symbolic value of Tobruk; an exchange of ambiguous signals between Churchill and Auchinleck led to the garrison being surrounded, rather than evacuated as intended. On 20 June the Panzerarmee Afrika attacked Tobruk with massed air support, penetrated a weak spot on the eastern defensive perimeter and captured the port. Much of the garrison on the western perimeter had not been attacked but they were cut off from their supplies and transport, without the means to escape from Tobruk. The majority had to surrender and 33,000 prisoners were taken.

The surrender was the second-largest capitulation by the British Army in the war, after the fall of Singapore in February 1942. The loss of Tobruk came as a severe blow to the British leadership and precipitated a political crisis in Britain. The United States expedited the dispatch of supplies and equipment to the Middle East. Rommel persuaded the Axis commanders that the supplies captured at Tobruk and the disorganised state of the British forces would enable the Axis easily to occupy Egypt and the Suez Canal. Operation Herkules, the Axis invasion of the island of Malta, was postponed and the Axis air forces instead supported the pursuit into Egypt, which suffered severe supply constraints as the Panzerarmee Afrika receded from its bases. The Axis advance was halted at the First Battle of Alamein in July 1942.

A British Court of Inquiry was held in absentia later in the year, which exonerated Klopper and ascribed the defeat to failures among the British high command. Only seven copies of the verdict were circulated, one being transmitted to General Jan Smuts on 2 October 1942. The findings were kept secret until after the war, which did little to restore the reputation of Klopper or his troops.

The small port of Tobruk in Italian Cyrenaica had been fortified by the Italians from 1935. Behind two old outlying forts, they constructed a novel fortification, consisting of a double line of concrete-lined trenches 54 km (34 mi) long, connecting 128 weapons pits protected by concealed anti-tank ditches but the fortifications lacked overhead protection and there was no defence in depth. Tobruk was captured by Australian forces in January 1941 during Operation Compass, the first large Allied military operation of the Western Desert Campaign. Following the arrival of the German Afrika Korps commanded by Erwin Rommel in Operation Sonnenblume in March, Axis forces retook much of the lost territory in Cyrenaica; Tobruk was cut off and besieged between April and December 1941.

Using the Italian defences, ill-organised attacks by Axis forces were defeated by the 30,000-strong Australian garrison (replaced in September by a British and Allied force), allowing time for the fortifications to be improved. The Allied occupation of Tobruk was a threat to the Axis communications. It denied them the use of the port and it tied down four Italian divisions and three German battalions (a force twice the size of the garrison). During 1941, supplied from the sea and surviving successive Axis assaults, the defence of Tobruk became a symbol of the British war effort. The relief of Tobruk was the object of Operation Brevity in May and Operation Battleaxe in June, both of which failed. Operation Crusader in November and December 1941 raised the siege and forced the Axis out of Cyrenaica into Tripolitania.

Supplied with more modern tanks, the second Axis offensive saw the reoccupation of western Cyrenaica but the Axis ran out of supplies to the west of Gazala. A lull followed as both sides prepared for a new offensive. The British built up the Gazala Line, fortified positions known as "boxes", defended by extensive minefields. The Axis forces forestalled the British with Unternehmen Venezia (known to the British as the Battle of Gazala), which began on 26 May 1942. Poorly armed and armoured British tanks and inferior co-ordination allowed Rommel to defeat the Eighth Army tanks piecemeal and by 13 June the British had begun to retreat eastwards from Gazala, leaving Tobruk vulnerable to attack.