The Palmyra offensive of May 2015 was a military operation launched during the Syrian Civil War by the Islamic State (IS) on May 13–26, 2015, in an attempt to capture the government-held Tadmur District of the Homs Governorate, including the administrative centre of Tadmur, known in English as Palmyra. The ruins and ancient monuments of Palmyra, which lie on the south-western fringe of the modern city, have been a UNESCO World Heritage Site since 1980. The ruins were part of a desert oasis that was one of the most significant cultural centers of the ancient world, linking the civilizations of Persia, India, China with the Roman Empire through trade. The offensive was one of the largest offensives launched by ISIL, the largest one conducted by ISIL in Syria since the 2014 Eastern Syria offensive, with the result of the offensive increasing ISIL's control of Syria to at least 50%.

On 13 May 2015, ISIL took advantage of the military's redeployment the day before to Idlib province, where ongoing fighting was taking place, and launched a two-pronged offensive towards Palmyra and its military sites, including Station 3 and the large weapon depots. The operation started on the outskirts of Al-Sukhnah, northeast of Palmyra. The militants captured al-Thinayya checkpoint at the northern entrance to the town, after which they advanced into major parts of Al-Sukhnah, with fighting taking place at the main police station, the headquarters of the Ba'ath Party and the National Hospital. This caused a state of panic among civilians in Palmyra and raised concerns about the possibility of ISIL attacking the nearby archaeological monuments. In the end, ISIL took control of Al-Sukhnah, giving them control over a strategic highway linking Homs with the Deir ez-Zor Governorate to the northeast.

After the capture of Al-Sukhnah, ISIL took all the Syrian Army posts between it and Palmyra. Fighting took place near the government security center of Hajjana and the officers' housing east of Palmyra, with ISIL taking control of several checkpoints, after hitting them with missiles. As they advanced to the outskirts of Palmyra, they seized all of the Amiriya area, north of the city, or at least its northern part, and the Muktab Al-Dour checkpoint in the eastern outskirts. ISIL also reportedly took a large government ammunition warehouse outside Palmyra, and was bombing a nearby government-controlled airport. The militants attempted to breach the eastern sector of the T-4 airbase but were repelled. According to a pro-government source, the Syrian Army later launched a counter-attack and recaptured Amiriya and the Muktab Al-Dour checkpoint. Before nightfall, the Syrian Army and the NDF launched a counterattack at Al-Sukhnah as well, and reportedly infiltrated ISIL positions at the southwestern corridor. In the evening ISIL seized Burj al-Ishara, a strategic high point near the historic Palmyra Castle. At the end of the day, ISIL was still in control of the Deir ez-Zor-Al-Sukhnah road, with the Syrian Army attempting to retake it.

The first day of fighting left 70 Syrian Army and 40 ISIL fighters dead, including the offensive's top ISIL commander. On 14 May, the Syrian Army shelled Burj al-Ishara, while ISIL made an attempt to break into the city of Palmyra. The jihadists managed to enter the neighbourhoods in the eastern outskirts of the city, but were expelled by government troops. After re-securing once again the Muktab Al-Dour checkpoint, government forces reportedly advanced to the village of Al-Basateen, where the eastern frontline remained that night. ISIL also targeted during the day the Tadmor Prison and the airport in an attempt to seize the airbase's major weapons depot. Elsewhere, government troops took full control of the Al-Hayl Gas Field, killing 32 militants, after which they proceeded to take Al-Thathah, Al-Fawl, Al-Mujawar, Al-Hawa, and Al-Sina'a. According to a military source, ISIL was still positioned one kilometer from Amiriya, and was in control of al-Sukhnah, but the Syrian Army remained at the outskirts of the village.

On 15 May, the Syrian Army dispatched reinforcements to Palmyra, while ISIL was 1 km (0.62 mi) from the Palmyra archeological site, but pulled back from the eastern outskirts to about 2 km (1.2 mi) where clashes were still erupting as the Syrian Army launched a counter-offensive. ISIL once again captured the northern part of Amiriya, killing a number of civilians, before they were pushed out again by heavy air-strikes. Two more attacks were launched against the air-base, with both being repelled. During the night, ISIL shifted its focus to the ruins but were unable to infiltrate them. Before dusk, Syrian Army units continued their counterattack at al-Sukhnah, reaching the town's outskirts. Reinforcements from the special forces unit, "Suqur al-Sahara," arrived to reinforce government forces.

On 16 May, ISIL fighters reached the ruins and captured parts of the historic citadel of Fakhr-al-Din al-Ma'ani as fighting continued. Later, ISIL once again seized Amiriya and continued its advance, taking the northern part of Palmyra after a massive attack.

By the morning of the next day, the Syrian Army had pushed ISIL back from Palmyra, fully securing the city. The Syrian Army also recaptured Palmyra castle and reportedly, the strategic Radio and Television Communication Hill (Burj al-Ishara) as well. Life in the town returned to normal with stores and businesses gradually reopening. However, fighting continued to the north in Amiriya and east of Palmyra, where ISIL captured the T-3 Pumping Station and al-Hayl gas field.

On 19 May, the SAA and NDF recaptured Amiriya. As the Army smashed its way into forward ISIL positions, they discovered piles of tactical vests, thermal missiles, stacks of Muslim prayer books in Russian (left by Chechen fighters) and enough ammunition for each rebel to carry 10,000 rounds.