The Alcobaça Monastery or Alcobasa Monastery (Portuguese: Mosteiro de Alcobaça, Mosteiro de Santa Maria de Alcobaça) is a Catholic monastic complex located in the town of Alcobaça (or Alcobasa, Portuguese pronunciation: [alkuˈβasɐ] ), in central Portugal, 120 km (75 mi) north of Lisbon and 110 km (68 mi) south of Coimbra.

The monastery was established in 1153 by the first Portuguese king, Afonso Henriques, and would develop a close association with the Portuguese monarchy throughout its seven-century-long history. This association led to the monastery becoming the richest and most influential in Portugal by 1300, with a population of almost 1,000 monks and business interests including farming, fishing and trade. It closed in 1834, amid the dissolution of the monasteries in Portugal.

The church and monastery were the first Gothic buildings in Portugal, and, together with the roughly older Augustinian Monastery of Santa Cruz in Coimbra, it was one of the most important mediaeval monasteries in Portugal. Due to its artistic, cultural and historical relevance, it was included in UNESCO's World Heritage Site list in 1989.