Line D is the only line not using the joint route between Estádio do Dragão and Senhora da Hora. It has a 4 km long tunnel between Polo Universitário and the bridge across the Douro river with 8 stations. For the river crossing the upper deck of the complete renovated Ponte Luís I is used. The upper deck of the bridge and the adjacent Avenida da República in VN de Gaia had electric trams from 1905 until 1959.
Line D crosses the joint route at Trindade. Here is also a 274 m long single track tunnel made between the tunnel of line D and the old tunnel of Trindade, which is now part of the joint route. This connecting tunnel is used to get the trams of line D from and to the depot. Line D has a total length of over 8 km and 16 stations/stops: Hôspital de São João – IPO – Polo Universitário – Salgueiros – Combatentes – Marquês – Faria Guimarâes – Trindade – Aliados – Sâo Bento – Jardim do Morro – General Torres – Câmara Gaia – João de Deus – Dom João II – Santo Ovídio.
17 September 2005 Polo Universitário – Trindade – Câmara Gaia (5.7 km)
10 December 2005 Câmara Gaia – João de Deus (0.4 km)
31 March 2006 Hôspital de São João – Polo Universitário (1.2 km)
28 May 2008 João de Deus – Dom João II (0.6 km)
15 October 2011 Dom João II – Santo Ovídio (0.5 km)
Travel time 25’.
Time-table (winter 2021/2022): mo-fr daytime every 6′, evening every 12′, weekends daytime every 10′, evening every 15′.