The siege of Sevastopol, also known as the defence of Sevastopol (Russian: Оборона Севастополя, romanized: Oborona Sevastopolya) or the Battle for Sevastopol (German: Schlacht um Sewastopol; Romanian: Bătălia de la Sevastopol; Russian: Битва за Севастополь, romanized: Bitva za Sevastopol'), was a military engagement that took place on the Eastern Front of the Second World War. The campaign was fought by the Axis powers of Germany and Romania against the Soviet Union for control of Sevastopol, a port in Crimea on the Black Sea. On 22 June 1941, the Axis invaded the Soviet Union during Operation Barbarossa, with Axis land forces reaching the Crimean peninsula in the autumn of 1941 and overrunning most of the area. The only objective not in Axis hands was Sevastopol. Several attempts were made to secure the city in October and November 1941. A major attack was planned for late November, but heavy rains delayed it until 17 December 1941. Under the command of Erich von Manstein, Axis forces were unable to capture Sevastopol during this first operation. Soviet forces launched an amphibious landing on the Crimean peninsula at Kerch in December 1941 to relieve the siege and force the Axis to divert forces to defend their gains. The operation saved Sevastopol for the time being, but the bridgehead in eastern Crimea was eliminated in May 1942.

After the failure of their first assault on Sevastopol, the Axis opted to conduct siege warfare until the middle of 1942, at which point they attacked the encircled Soviet forces by land, sea, and air. On 2 June 1942, the Axis began this operation, codenamed Störfang (Sturgeon Catch). The Soviet Red Army and Black Sea Fleet held out for weeks under intense Axis bombardment. The German Air Force (Luftwaffe) played a vital part in the siege, its 8th Air Corps bombing the besieged Soviet forces with impunity, flying 23,751 sorties and dropping 20,528 tons of bombs in June alone. The intensity of the German airstrikes was far beyond previous German bombing offensives against cities such as Warsaw, Rotterdam or London. At the end of the siege, there were only 11 undamaged buildings left in Sevastopol. The Luftwaffe sank or deterred most Soviet attempts to evacuate their troops by sea. The German 11th Army suppressed and destroyed the defenders by firing 46,750 tons of artillery ammunition on them during Störfang.

Finally, on 4 July 1942, the remaining Soviet forces surrendered and the Germans seized the port. The Soviet Separate Coastal Army was annihilated, with 118,000 men killed, wounded or captured in the final assault and 200,481 casualties in the siege as a whole for both it and the Black Sea Fleet. Axis losses in Störfang amounted to 35,866 men, of whom 27,412 were German and 8,454 Romanian. With the Soviet forces neutralized, the Axis refocused their attention on the major summer campaign of that year, Case Blue and the advance to the Caucasus oilfields.

The Soviet naval base at Sevastopol was one of the strongest fortifications in the world. Its site, on a deeply eroded, bare limestone promontory at the southwestern tip of the Crimea, made an approach by land forces exceedingly difficult. The high-level cliffs overlooking Severnaya Bay protected the anchorage, making an amphibious landing just as dangerous. The Soviet Navy had built upon these natural defenses by modernizing the port and installing heavy coastal batteries consisting of 180mm and 305mm re-purposed battleship guns which were capable of firing inland as well as out to sea. The artillery emplacements were protected by reinforced concrete fortifications and 9.8-inch thick armored turrets.

The port was a valuable target. Its importance as a potential naval and air base would enable the Axis to conduct far-ranging sea and air operations against Soviet targets into and over the Caucasus ports and mountains. The Red Air Force had been using the Crimea as a base to attack targets in Romania since the Axis invasion in June 1941, proving its usefulness as an air base. Likewise, the Wehrmacht had launched a bombing raid on the Sevastopol naval base at the start of the invasion.

Since the beginning of Barbarossa, the offensive against the USSR had not really addressed Crimea as an objective. German planners assumed the area would be captured in mopping-up operations once the bulk of the Red Army was destroyed west of the Dnieper river. But in June, attacks by Soviet aircraft from Crimea against Romania's oil refineries destroyed 12,000 tons of oil. Hitler described the area as an "unsinkable aircraft carrier" and ordered the conquest of Ukraine and Crimea as vital targets in Directive 33, dated 23 July 1941.

The Command of the Army (OKH) issued orders that the Crimea was to be captured as soon as possible to prevent attacks on Romanian oil supplies, vital to the German military. Hitler, impatient with obstruction to his commands to advance in the south, repeated on 12 August his desire that the Crimea be taken immediately. Over a month later, during the capture of Kiev, Generaloberst (General Colonel) Erich von Manstein was given command of the German 11th Army on 17 September. After only a week in command, he launched an assault upon Crimea. After severe fighting, Manstein's forces defeated several Soviet counteroffensives and destroyed two Soviet armies. By 16 November, the Wehrmacht had cleared the region, capturing its capital Simferopol, on 1 November. The fall of Kerch on 16 November left only Sevastopol in Soviet hands.

By the end of October 1941, Major-General Ivan Yefimovich Petrov's Independent Coastal Army, numbering 32,000 men, had arrived at Sevastopol by sea from Odessa further west, it having been evacuated after heavy fighting. Petrov set about fortifying the inland approaches to Sevastopol. He aimed to halt the Axis drive on the port by creating three defence lines inland, the outermost arc being 16 km (10 mi) from the port itself. Soviet forces, including the Soviet 51st Army and elements of the Black Sea Fleet, were defeated in the Crimea in October and were evacuated in December, leaving Petrov's force as Sevastopol's main defence force. Having cleared the rest of the Crimea between 26 September – 16 November, the Romanian 3rd Army and German 11th Army prepared for an attack on the port. The German 11th Army was the weakest on the entire front, initially containing only seven infantry divisions. The Romanians contributed a large force, but were only lightly equipped and generally lacked heavy artillery. The weather turned against the Axis in mid-October and torrential downpours delayed the buildup. This gave Vice Admiral Filipp Oktyabrsky, commander of the Black Sea Fleet, time to bring in men and materiel from Novorossiysk. By 17 December, the weather had cleared sufficiently for the Axis to begin a major operation.