HELSINKI—Finland has approved the building of a controversial Russian-backed nuclear power plant, strengthening energy ties between Helsinki and Moscow at a time when many western capitals are seeking to isolate Russia over its intervention in Ukraine.

Finnish lawmakers on Friday voted by 115 to 74 in favor of the government’s decision to approve a plan submitted by a Finnish-Russian consortium Fennovoima Oy to build a new nuclear power plant on the northwest coast of Finland at an estimated cost of between €4 billion ($4.9 billion) and €6 billion. Russian nuclear power company Rosatom holds a 34% minority stake in the group and will raise finance for the new plant.

Despite the large vote in favor, Fennovoima’s reliance on Russian backing has stirred unease among some Finnish politicians and the public at large, many of whom see the deal serving Russian geopolitical interests at a time of heightened tensions. Eyebrows are also likely to be raised in other European countries.

Ville Niinistö, the leader of the Greens of Finland, which opposes nuclear power on principle, said Russia can use Fennovoima to create a perception, especially before its own people, that Moscow hasn’t been isolated because of its Ukraine policy.

Earlier this autumn, Mr. Niinistö withdrew his party from the Finnish government in protest over the project, leaving the administration with a razor-thin majority in Parliament.