Johan Casimir Ehrnrooth (Russian: Казимир Густавович Э́рнрот, Kazimir Gustavovich Ernrot; 26 November 1833 – 5 February 1913) was a Finnish statesman in the service of Imperial Russia, who also acted as Prime Minister of Bulgaria.

Ehrnrooth was born to an affluent noble family in the Seesta Mansion [fi] in Nastola in the Grand Duchy of Finland.

At the time a Major in the Russian Army, Ehrnrooth continued to rise through the ranks in campaigns against Polish rebels and fighting to remove the Ottoman Turks from Bulgaria.

Ehrnrooth became the strongman of the government, and became Prime Minister on 9 May 1881 whilst Alexander finalised his plans to assume full control of the country.

After his return to Russia Ehrnrooth became Minister-Secretary of State for Finnish Affairs, although the job, which involved drives towards Russification,[citation needed] did not suit the Finn.