Reinosa is a municipality in Cantabria, Spain. As of 2009, it has 10,307 inhabitants. The municipality, one of the smallest by land area in Cantabria, is notable for being one of the nearest towns to the headwaters of the Ebro River. It is completely surrounded by the municipality of Campoo de Enmedio and was created a city by King Alfonso XIII in 1927, being one of only three urban centres in Cantabria with that honour, the others being Santander and Torrelavega.

Reinosa is the largest town in this area of Cantabria and is on the main railway line and located close to the A67 motorway. The town contains a number of hotels and restaurants, and Montesclaros High School is located here. The source of the River Ebro is in the village of Fontibre where a stream emerges from the ground 3 kilometres (2 mi) from the town; a large artificial lake Embalse del Ebro was created in the Franco era by damming the river just below Reinosa. This is one of the largest reservoirs in northern Spain and is mainly filled in spring as a result of the melting of the winter snow in the mountains. The lake is the uppermost of many reservoirs on the Ebro.