The northern end of Unioninkatu from the Kaisaniemi roundabout to Pitkäsilta is a busy traffic route, and forms a part of a road leading to Hakaniemi and then to Hämeentie together with Kaisaniemenkatu and Siltasaarenkatu.
[2]: 18 When Johan Albrecht Ehrenström made a new zoning plan for Helsinki that had been chosen as the new capital of Finland in 1812, Unioninkatu became a key street running straight from Pitkäsilta to the south, to the Ulriikanporinvuori hill, now known as Tähtitorninvuori.
[4] The Stockmann department store was located on the street in the so-called Kiseleff House, which later contained only offices, but which now hosts a shopping centre.
In May 2024, Dimitri Qvintus, an assistant to the Prime Ministers Antti Rinne and Sanna Marin proposed renaming Unioninkatu to "Euroopan unionin katu" ("European Union Street").
[5] Buildings on the southern part of Unioninkatu along the Tähtitorninvuori park include the German Church in Helsinki and the Svenska normallyceum i Helsingfors school.
Slightly to the north Unioninkatu also runs along the western edge of the Senate Square, where the main building of the University of Helsinki is located on the street.
After this there are hospital buildings on either side of Unioninkatu, including Topelia and the Old and New Clinics, which are now in use of the university and form part of the City Centre Campus.