Posiva said the contract covers the excavation work for two central tunnels and five deposition tunnels connected to them.
The work will start towards the end of 2019 and is expected to take about two-and-a-half years.
Posiva announced in June that it was beginning construction of a €500m encapsulation plant and final disposal facility for spent nuclear fuel at Olkiluoto, making Finland the first country to begin construction of a final repository.
Posiva is responsible for the final disposal of used nuclear fuel generated by Teollisuuden Voima Oyj (TVO) at its nuclear station in Olkiluoto and Fortum at its nuclear station in Loviisa. TVO owns 60% of Posiva and Fortum owns 40%.
In Finland, full lifecycle management of nuclear fuel is a precondition for the production of nuclear electricity.