Puente de San Juan, San Martín de Valdeiglesias, Medieval bridge in San Martín de Valdeiglesias, Spain.

The Puente de San Juan is a medieval bridge that crosses the Alberche River using four stone arches and sits near the Picadas Reservoir. The structure spans the water on a straightforward path that connects both riverbanks.

The bridge was built in 1459 by order of Alonso Matamoros, abbot of the Valdeiglesias monastery, replacing an earlier river crossing by boat. It linked the Arenas de San Pedro and La Vera trade routes with Madrid. Repairs, including one directed by engineer Luis de Rute Giner in 1867, followed growing traffic. Traffic was diverted in 1973 to the parallel Picadas viaduct.

The bridge takes its name from a chapel dedicated to Saint John the Baptist that stood on the small plain where it was built. Until 1973 it carried the road between Pelayos de la Presa and Navas del Rey; traffic now crosses the parallel Picadas viaduct, and the old span is used by walkers and bathers heading to the San Juan reservoir, the only one in the Madrid region where swimming is allowed.

The bridge was closed to motor traffic in 1973, so it is now reached on foot from the M-501; the road crosses the river on the parallel Picadas viaduct instead. The deck is 50 m long and 5 m wide, with iron railings. The river here is the tail of the Picadas reservoir, at about 520 m above sea level.

The bridge lost most of its ashlar blocks over time; today they survive only on the southern side, while the rest is rubble masonry and reinforced concrete. Its cutwaters are rounded, and the 50-metre deck was widened with a cantilever to 5 metres.