Lajjun or Lejjun (Arabic: اللجّون, el-Lejjūn) was a large Palestinian Arab village located 16 kilometers (9.9 mi) northwest of Jenin and 1 kilometer (0.62 mi) south of the remains of the biblical city of Megiddo. The Israeli kibbutz of Megiddo was built 600 metres north-east of the depopulated village on the hill called Dhahrat ed-Dar starting from 1949.

The initial settlement grew next to a Roman legion camp, known simply as "Legio", used by the Legio VI Ferrata, for which it provided services. Named after the camp, Lajjun's history of habitation spanned some 1,800 years, from the 2nd century during the Roman province of Syria Palaestina, to the 20th century. Under Abbasid rule it was the capital of a subdistrict, during Mamluk rule it served as an important station in the postal route, and during Ottoman rule it was the capital of a district that bore its name. After the collapse of the Ottoman Empire towards the end of World War I, Lajjun and all of Palestine was placed under the administration of the British Mandate. The village was depopulated during the 1948 Arab–Israeli War, when it was captured by Israel. Most of its residents subsequently fled and settled in the nearby town of Umm al-Fahm.

The name Lajjun derives from the Roman name Legio, referring to the Roman legion stationed there. In the 3rd century, the town was renamed Maximianopolis ("City of Maximian") by Diocletian in honor of Maximian, his co-emperor, but the inhabitants continued to use the old name. Under the Caliphate, the name was Arabicized into al-Lajjûn or el-Lejjûn, which was used until the Crusaders conquered Palestine in 1099. The Crusaders restored the Roman name Legio, and introduced new names such as Ligum and le Lyon, but after the town was reconquered by the Muslims in 1187, al-Lajjun once again became its name.

Modern Lajjun was built on the slopes of three hills, roughly 135–175 meters above sea level, located on the southwestern edge of the Jezreel Valley (Marj ibn Amer). Jenin, the entire valley, and Nazareth range are visible from it. The village was located on both the banks of a stream, a tributary of Kishon River. The stream flows to the north and then east over 6 kilometres (3.7 mi) before arriving at Lajjun. That section is called Wadi es-Sitt (valley of the lady) in Arabic, The northern quarter was built in close proximity to a number of springs, including 'Ayn al Khalil, 'Ayn Nasir, 'Ayn Sitt Leila, and 'Ayn Jumma, collectively known as 'Uyun Seil Lajjun. The eastern quarter was next to 'Ayn al Hajja. From Lajjun onward the stream is called Wadi al-Lajjun in Arabic. In Hebrew, the Israeli Government Naming Committee decided in 1958 to use the name Nahal Qeni (Hebrew: נַחַל קֵינִי) for the entire length of the stream, based on its ancient identification (see below). Lajjun is bordered by Tall al-Mutsallem to the northeast, and by Tall al-Asmar to the northwest. Lajjun, which was linked by secondary roads to the Jenin-Haifa road, and the road that led southwest to the town of Umm al-Fahm, laid close to the junctions of the two highways.

Nearby localities included, the destroyed village of Ayn al-Mansi to the northwest, and the surviving villages of Zalafa to the south, Bayada and Musheirifa to the southwest, and Zububa (part of the Palestinian territories) to the southeast. The largest town near al-Lajjun was Umm al-Fahm, to the south.

Lajjun is about 1 kilometer (0.62 mi) south of Tel Megiddo, Arabic name: Tell al-Mutasallim, which is identified with ancient Megiddo. During the rule of the Canaanites and then the Israelites, Megiddo, located on the military road leading from Asia to Egypt and in a commanding situation, was heavily fortified by both peoples.

Lajjun stream has been identified with the brook Kina, or Qina, which is mentioned in the Egyptian descriptions of Thutmose III's Battle of Megiddo. According to the reconstruction of Harold Hayden Nelson, the entire battle was fought in the valley, between the three quarters of modern Lajjun. However, both Na'aman and Zertal suggested alternative locations for Qina. This stream may be the "Waters of Megiddo" in the Song of Deborah In the same context, Judges 4 attests to the presence of a branch of the Kenite clan somewhere in the area; relating this name to Thutmose's Annals, scholars like Shmuel Yeivin theorized that the name Qina derives from qyni (Hebrew: קיני). Donald B. Redford noted that the Egyptian transliteration might be of "qayin".

Modern-day historical geographers have placed the Second Temple period village of Kefar ʿUthnai (Hebrew: כפר עותנאי) in the confines of the Arab village, and which place-name underwent a change after a Roman Legion had camped there. It appears in Latin characters under its old name Caporcotani in the Tabula Peutingeriana Map, and lay along the Roman road from Caesarea to Scythopolis (Beit Shean). Ptolemy (Geography V, 15: 3) also mentions the site in the second century CE, referring to the place under its Latin appellation, Caporcotani, and where he mentions it as one of the four cities of the Galilee, with Sepphoris, Julias and Tiberias. Among the village's famous personalities was Rabban Gamliel. After the Bar Kochba Revolt—a Jewish uprising against the Roman Empire—had been suppressed in 135 CE, the Roman emperor Hadrian ordered a second Roman legion, Legio VI Ferrata (6th "Ironclad" Legion), to be stationed in the north of the country to guard the Wadi Ara region, a crucial line of communication between the coastal plain of Palestine and the Jezreel Valley. The place where it established its camp was known as Legio.