| Johan Casimir Ehrnrooth | |
| Order: | 5th |
| Office: | Prime Minister of Bulgaria |
| Term Start: | 9 May 1881 |
| Term End: | 13 July 1881 |
| Predecessor: | Petko Karavelov |
| Successor: | Vacant |
| Order2: | War Minister of Bulgaria |
| Term Start2: | 17 April 1880 |
| Term End2: | 13 July 1881 |
| Predecessor2: | Alexander Timler |
| Successor2: | Vladimir Krylov |
| Order3: | Minister of Interior |
| Term Start3: | 9 May 1881 |
| Term End3: | 13 July 1881 |
| Premier3: | Himself |
| Predecessor3: | Petko Slaveykov |
| Birth Place: | Seesta Mansion, Nastola, Grand Duchy of Finland, Russian Empire |
| Death Date: | (aged 79) |
| Death Place: | Helsingfors (now Helsinki, Finland), Grand Duchy of Finland, Russian Empire |
| Allegiance: | |
| Branch: | |
| Rank: | Lieutenant-General |
| Commands: | Vitebsk Infantry Regiment 11th Infantry Division Ministry of War of Bulgaria |
| Battles: | Caucasian War, January Uprising, Russo-Turkish War (1877–78) |
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 in Nastola in the Grand Duchy of Finland. In 1856, he graduated from the Imperial Military Academy in Saint Petersburg and enlisted in the Imperial Russian Army.
Ehrnrooth first came to prominence when he played a leading role in suppressing the resistance of Imam Shamil and the Caucasian Avars in 1859. 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. Following the Independence of Bulgaria Ehrnrooth was chosen by Russia to look after the interests of Alexander of Bulgaria, becoming Minister of War on 17 April 1880. 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. Ehrnrooth became the strongest supporter of Alexander during this period, although he was forced to leave Bulgaria when the experiment floundered.
After his return to Russia Ehrnrooth became Minister-Secretary of State for Finnish Affairs, although the job, which involved drives towards Russification, did not suit the Finn. He retired in the 1890s and died of a stroke in Helsinki at the age of 79. Although he had no children both Finnish World War II colonel and later general Adolf Ehrnrooth and former chairman of Nokia and Kymmene Corporation Casimir Ehrnrooth are from his family line.