Riserva Naturale Pian di Spagna e Lago di Mezzola, Regional nature reserve in Province of Sondrio, Italy.

The Riserva Naturale Pian di Spagna e Lago di Mezzola is a regional nature reserve in Lombardy, sitting between Lake Como and Lake Mezzola. It covers wetlands, alluvial plains, and open water areas broken up by flat meadows and small wooded rises.

The area was placed under protection in 1985 after its wetlands were recognized as important for wildlife. Long before that, the plain had been shaped by the Adda and Mera rivers, which repeatedly flooded and reshaped the land over centuries.

The name Pian di Spagna, meaning "Plain of Spain", comes from the Spanish domination of the 16th to 18th centuries, when the plain marked the border between the Grisons, who held the Valtellina, and the Duchy of Milan under the Spanish crown. The plain's strategic position led to fortifications, including the Forte di Fuentes on the northern hill of Montecchio, built by the Count of Fuentes, governor of Milan, and linked to the Torre di Sorico, the Forte d'Adda and the towers of Curcio and Fontanedo. The fort was dismantled by Napoleon in 1796; its perimeter walls and buildings survive and have been managed since 2012 by the Museo della Guerra Bianca.

The reserve can be explored on foot or by bike along marked paths, with some sections near the water's edge being easier to reach than others. Birdwatchers tend to visit in the early morning hours, when wildlife is most active along the shore.

Lake Mezzola and Lake Como were once directly connected and formed a single body of water before river sediment gradually separated them over centuries. The flat land between the two lakes is what remains of that slow natural separation, and you can see it clearly as you walk through the reserve.