Construction work on the new metro line is set to begin at the end of 2009.
The westward expansion of the metro line was approved late Wednesday evening.
The Helsinki City Council's approval of the Ruoholahti to Matinkylä line sealed the deal.
Espoo's City Council already approved the construction on Monday.
Building costs of the new line are estimated around 714 million euros. Espoo's share of the costs is 355 million euros.