The Luí­s I Bridge (Ponte Luí­s I) is a metal arch bridge that spans the Douro River between the cities of Porto and Vila Nova de Gaia. It was projected by the disciple and co-worker of Gustave Eiffel: the Engineer Teófilo Seyring.
The construction was begun in 1881 and the bridge opened on 31 October 1886.

This bridge is a representative example of iron architecture and techniques.
At the time of construction its span of 172 m was the longest of its type in the world. Its total length is 385,25 m and 44,6 m in arch heigh.
Originally built to take road traffic on both decks, at various times it saw trams on the upper and trolleybuses on the lower. The top deck is now occupied by Line D of the Metro do Porto and a pedestrian walkway.

The bridge name is “Luí­s I” and not “Dom Luí­s I”. This is because the king “Dom Luí­s I” did not come to the inauguration and the population decided to take off “Dom” to the name.

Teófilo Seyring projected also the D. Maria Pia Bridge. The D. Maria Pia bridge (Ponte D. Maria Pia) is a railway bridge inaugurated in 1877. It was in railway service during 114 years, before a modern bridge was constructed in 1991 – S. João Bridge.