As president, Niinistö has enjoyed broad support across most of the Finnish political spectrum, including for his handling of ties with Russia and its attack on Ukraine.
The nationalist Finns Party, led by Finance Minister Riikka Purra, is now the second largest in Parliament, while the once-mighty Centre has fallen to fourth place.
The outgoing finance minister will not follow fellow cabinet members Sanna Marin and Maria Ohisalo in stepping down as party leaders following the election.