Fjärdingen is a neighbourhood in the inner city of Uppsala, Sweden, situated on the western banks of the river Fyris. It has been the ecclesiastical centre of Sweden since the 13th century and contains many of the historical sights and landmarks of Uppsala, notably Uppsala Cathedral on the Domberget hill, with the Archbishop's residence and Church of Sweden offices, as well as the historical university district of Sweden's oldest university, Uppsala University. Notable university buildings in the area include University Hall, Gustavianum and Carolina Rediviva, as well as most of the student nation buildings. To the south, Uppsala Castle, the University Hospital and the Stadsträdgården city park form the boundary of the historical inner city.

Fjärdingen's natural border to the north-east is the River Fyris; to the south-east the neighbourhood is limited by Sjukhusvägen and the southern end of the city park, to the south-west by Dag Hammarskjölds väg and Kyrkogårdsgatan and to the north-west by Skolgatan.

There are several bridge crossings over to the modern city centre. Further south along the opposite side of the river, to the south-east of Fjärdingen, is Kungsängen. To the south of Uppsala Castle and the University Hospital along the river is Polacksbacken. To the south-west is Kåbo and to the north-west is Luthagen. The toponym Främre (Near) Luthagen is sometimes applied to the area between Luthagsesplanaden and Skolgatan.

In the early 19th century, the entirety of the city of Uppsala used to encompass the three inner city neighbourhoods of Fjärdingen west of the river, Svartbäcken east of the river north of Stora torget and Kungsängen to the south of Stora torget. Today, the toponyms Svartbäcken and Kungsängen are mainly used for the neighbourhoods further to the north and south of Stora torget, respectively, as the city centre around Stora torget is considered an administrative subdivision in its own right, also unofficially known as Dragarbrunn.

Uppsala Cathedral on Domberget, the cathedral hill, is the city's main landmark since its construction in the 13th century and the seat of the Primate of the Lutheran Church of Sweden, the Archbishop of Uppsala. The remains of the medieval fortifications surrounding the cathedral have been incorporated into later buildings, partially encircling the cathedral. Across from the main western entrance to the cathedral is Gustavianum, the former main building of Uppsala University which today remains in use by the university as a lecture building and university museum. The Gustavianum cupola contains a restored baroque Anatomical theatre, the second oldest remaining of its kind in the world.

Drottninggatan is the main avenue and shopping street in the area, leading southwest from the modern city centre across River Fyris and past Fyristorg uphill to the university library, Carolina Rediviva. The final climb uphill through the park landscape to the library is commonly referred to as Carolinabacken, the site of yearly student festivities during the traditional Valborg celebrations on April 30.

Sysslomansgatan/Västra Ågatan are the main north-south business streets, running parallel to the river past Fyristorg and joining at Sankt Olofsgatan. The former University Mill building presently houses a restaurant and the Upplandsmuseet regional museum. The triangular Sankt Eriks torg north of the cathedral is home to Uppsala's market hall. The southern end of Sysslomansgatan at the intersection with Sankt Olofsgatan has been the site of Uppsala's oldest café, Ofvandahls, since 1878.

The University Park with the monument to Erik Gustaf Geijer is situated centrally in the neighbourhood, a short distance west of the cathedral. The main building of the university and the park was created in the 19th century on the site of the former Archbishop's Castle and the baroque horse riding courts. University Hall, Gustavianum, the Archbishop's Residence, Södermanlands-Nerikes nation, Smålands nation and the Church of Sweden office building are situated around the park. Other student societies in the area between Sankt Olofsgatan and Skolgatan north of the University Park include Uplands nation, Västmanlands-Dala nation and Kalmar nation.