The Battle of Porta (Greek: Μάχη της Πόρτας) was fought on 8–9 June 1943 at the Porta and Mouzaki passes in western Thessaly, between the partisans of the Greek People's Liberation Army (ELAS) and the Royal Italian Army, during the Axis occupation of Greece.

In spring 1943, a wave of successes demonstrated the rise of the Greek Resistance against the Axis powers, particularly the Italians; large areas of the mountainous interior of the country were effectively liberated. In response, the Italian 11th Army planned a large-scale anti-partisan operation for June. Warned of Italian intentions, ELAS General Headquarters withdrew its dispersed detachments to the central Pindus massif.

In Thessaly the forces withdrawn from the Mount Pelion and Mount Kissavos areas were detailed to watch the Porta and Mouzaki passes, covering the withdrawal of the rest of the Thessalian partisans. Contrary to orders from ELAS GHQ to only engage in disruptive hit-and-run attacks against the Italian army, the commanders of these two detachments, numbering around 250 men with mostly light weaponry and scarce ammunition, decided to hold the passes against the expected Italian attack. Over two weeks, field works were erected in front of the passes and outposts established in the villages. The Italians moved against the Greek positions on 8 June, with around 4,000 men, both infantry and cavalry, from the 24th Pinerolo Infantry Division, with artillery and aviation support.

Facing far superior numbers and lacking sufficient ammunition, the partisans were able to hold back the Italians at Porta on the first day, but at Mouzaki, the Italians forced them back and occupied the village. After reinforcements were brought in during the night, the Italians managed to advance on both flanks on 9 June, threatening to encircle the partisan position at Porta. The ELAS partisans withdrew to the mountains, but their actions had been successful in that the Italians, having suffered significant casualties, broke off their planned anti-partisan sweeps in the mountainous interior without continuing their advance.

On 6 April 1941, following a botched Italian invasion in October 1940, Nazi Germany invaded Greece through Bulgaria and Yugoslavia. The Greek capital Athens fell on 27 April, and by June, after the capture of Crete, all of Greece was under Axis occupation. Most of the country was left to the Italian forces, while Bulgaria annexed northeastern Greece and German troops occupied the strategically most important areas. A collaborationist government was installed, but its legitimacy among the Greek people was minimal, and its control over the country compromised by the patchwork of different occupation regimes Greece was divided into. As early as the autumn 1941, the first stirrings of a resistance movement were registered, with attacks on isolated Gendarmerie stations in Macedonia in northern Greece. The establishment of large-scale resistance organizations in 1942, most notably the communist-dominated National Liberation Front (EAM) and its military wing, the Greek People's Liberation Army (ELAS) began to challenge not only the collaborationist government's organs, but also the Italian occupation troops.

The winter and early spring of 1943 saw a series of resistance successes against the Italians in the mountainous areas of mainland Greece, with battlefield victories such as at Fardykambos, or the liberation of towns like Karditsa (12 March), Grevena (24 March), and Metsovo (22 April). By 16 April an Italian report noted that "control throughout the north-east, centre and south-west of Greece remains very precarious, not to say nonexistent". As a result, the high command of the Italian army in Greece, the 11th Army in Athens, decided to mount a large-scale and concerted anti-partisan effort, aimed at hemming in the partisan forces in the Pindus mountain massif, and then launching coordinated and concentric attacks in Thessaly, Central Greece, and Epirus to clear the area. The operation would begin after 20 May, the deadline set by the new collaborationist government under Ioannis Rallis for partisans to surrender in exchange for a full amnesty.

Consequently, the ELAS Central Committee issued orders to its regional headquarters to prepare for the attack, gathering the bulk of its forces in the Pindus massif, and relocating itself there, while leaving behind rear guards to obstruct the Italians. Central Committee member Kostas Karagiorgis informed the Thessaly Headquarters of ELAS of these decisions when it met at Porta on 17 May. Thus the partisan groups of western and eastern Thessaly—the Agrafa sub-command (Ὑπαρχηγείον Ἀγράφων), the Koziakas [el] sub-command (Ὑπαρχηγείον Κόζιακα), the Kissavos–Mavrovouni sub-command (Ὑπαρχηγείον Κισσάβου-Μαυροβουνίου) and Pelion–Karantaou sub-command (Ὑπαρχηγείον Πηλίου-Καραντάου)—were ordered to move towards the area of Mount Smolikas, while leaving behind some forces to harass the Italians, and occupying some strategic passes, i.e, the Porta–Mouzaki pass and the Kalambaka–Diava pass. After the Italian operation began on 22 May, the eastern Thessalian forces—the Kissavos and Pelion sub-commands—were forced to cross the Thessalian plain in a three-day operation, moving only under cover of darkness.

The Porta Pass is formed by the Portaikos [el] river as it descends from the Pindos mountains to the Thessalian plain. The village of Porta lies exactly at the entrance of the pass. In the 1940s, a narrow car road leading from Trikala to Pertouli passed near the village and crossed the river on a concrete bridge. The northern side of the pass, towards Koziakas, is nearly inaccessible except for narrow footpaths, but the southern side towards Agrafa is easier to traverse. The rear of the Porta defensive position could be attacked via the car road passing through the gap at Mouzaki allowed easy access to the village of Vatsounia. As a result, any defender of the Porta Pass against an opponent coming from the plain was also obliged to keep the Mouzaki–Porti area also under control to avoid being flanked.