The Dom Luís I (or Luiz I) Bridge (português. Ponte Luís I or Luiz I) is a double-decked metal arch bridge that spans the Douro River between the cities of Porto and Vila Nova de Gaia in Portugal. At the time of construction its span of 172 m was the longest of its type in the world.
The government held a competition for the construction of a metallic bridge over the Douro River on a site that was adjacent to an existing bridge that it would replace. Téophile Seyrig had engineered the D. Maria Pia Bridge project nearby, whilst working as a partner of Gustave Eiffel. He now took sole responsibility for the new, major Luís I Bridge. The construction was begun in 1881 and the bridge opened on 31 October 1886 (the upper deck; the lower deck opened in 1887).
Originally and for more than a century, the bridge carried road traffic on both decks. Along with other vehicles, electric trams crossed the upper deck from 1908 until May 1959, and trolleybuses crossed both decks from May 1959 until 1993. In 2003, the top deck was closed to private motor traffic and since that time the top deck has been occupied by Line D of the Metro do Porto light rail system (which opened in 2005) and a pedestrian walkway, offering great views on the historic center of Porto, the Port wine caves of Vila Nova de Gaia and the river Douro.
In 1982, the bridge was designated a cultural heritage "Property of Public Interest" (Imóvel de Interesse Público) by IGESPAR, the Institute for the Management of Architectural and Archaeological Heritage, a federal agency.
The bridge often is confused with the nearby Maria Pia Bridge, a railway bridge built nine years earlier and located a kilometre to the east. However, although they bear a strong resemblance to one another, the earlier bridge has only one deck.