Fuerte Reina Luisa, Colonial fort in Osorno, Chile.

Fuerte Reina Luisa is a fort in Osorno built on the bank of the Rahue River with a triangular shape and three bastions protecting the structure. The walls create a compact defensive system that controlled river passage and guarded the city behind it.

Tomás de Figueroa took possession of Osorno in November 1792 and ordered a fort on the Rahue, then called the river of canoes, against pirates and Mapuche attacks during the War of Arauco. In August 1793 a detachment of 60 men under sublieutenant Julián Pinuer Zurita was installed; Captain Manuel Olaguer Feliú of the Royal Corps of Engineers designed the work, finished in June 1794 with cancagua stone taken from the ruins of the old city and labour of convicts brought from Valdivia.

Ambrosio O'Higgins named the fort after María Luisa de Parma, wife of King Charles IV of Spain; it was first called San Luis because digging of the moat began on Saint Louis's day, 25 August. During the independence wars it was renamed Fuerte Mackenna after Chilean forces under Jorge Beauchef took it on 25 February 1820.

The fort stands on Calle Tomás de Figueroa, beside the access to the San Pedro bridge and about 250 m from where the Rahue meets the Damas. The 2007 renovation added perimeter fencing, lighting and a riverside walk, so the site can be reached on foot along the Rahue embankment. The exhibition hall also hosts temporary shows, talks and workshops, so check locally before planning a visit around a specific event.

The fort's alarm bell is kept at the Museo Histórico Municipal in Osorno. The 2007 restoration also replaced the artillery pieces with pedrero-type cannons and added perimeter fencing and lighting, at a cost of about 400 million pesos.