With all votes counted by the Statistics Office on Saturday, the left-wing Social Democrats won 13 of 27 seats up for grabs in the 81-seat upper house. Together with the far-left Communists, the two leftist parties hold a majority of 48 seats.
Prime Minister Petr Necas' conservative Civic Democrats won four seats for a total of 15, the lowest number ever.
The election was held over Friday and Saturday.
Although Parliament's lower house dominates the legislative process, the Senate can delay approval of spending cuts. It also plays an important role in passing constitutional amendments and approving judges for the Constitutional Court.