Kaarlo Castrén (28 February 1860 – 19 November 1938) was a Finnish politician and Prime Minister of Finland.
He represented the National Progressive Party.
Castrén was born in Turtola and graduated in 1887 as a Bachelor of Law.
From 1888 to 1892, he worked in the finance division of the Senate of Finland, and from 1888 to 1898 in the Castrén & Snellman attorneys-at-law office.
From 1892 to 1904, Castrén was a member of the board in the Kansallis-Osake-Pankki bank.
He attended the state board meetings in 1894 and from 1905 to 1906.
He was Senator from 1908 to 1909, after which he founded an attorney-at-law office.
In 1916, Castrén was elected as the director of Kansallis-Osake-Pankki.
In November 1918, Castrén was named Minister of Finance.
He served as Prime Minister of Finland from 17 April to 15 August 1919.
His government gave a proposal of the republic form of government in Finland, and after the proposal was accepted, the government disbanded.
Castrén died in Helsinki.