The Pont-y-Cafnau (Welsh for 'bridge of troughs'; sometimes written Pont y Cafnau or Pontycafnau), is a 14.2-metre (47 ft) long iron truss bridge over the River Taff in Merthyr Tydfil, Wales.

The bridge was designed by Watkin George and built in 1793 for his employer, the Cyfarthfa Ironworks, to support both a tramway and an aqueduct to carry limestone and water into the works.

A Grade II* listed building and Scheduled Ancient Monument, the Pont-y-Cafnau is the world's earliest surviving iron railway bridge.

Wooden uprights were attached to the iron risers that passed through the apex of each of the A-frames and these supported the upper trough carrying the Taff Fawr leat.

Reflecting its designer's roots in carpentry, the members composing the trusses are joined by dovetail and mortise and tenon joints commonly used in wooden structures.