Carolus Juhani Lassila (5 August 1922 Helsinki – 26 October 1987 Atlas Mountains, Morocco) was a Finnish diplomat.
Lassila's parents were Armas Lassila (1890–1955) and Martha Sofia Holmberg (1888–1975).
His uncles were Professor of Anatomy Väinö Lassila and Professor of Forest Technology Ilmo Lassila.
Carolus Lassila was married to Britta Synnöve Juliana Wilskman (1923–1987).Lassila was an expert in Arabic language and Arab culture.
He was an Ambassador to Beirut, Lebanon, and at the same time to Kuwait and Riyadh, Saudi Arabia from 1968 to 1974, Jeddah, Saudi-Arabia and to Abu Dhabi, United Arab Emirates between 1975 and 1977 and most recently Ambassador to Madrid, Spain as well as Morocco 1984–1987.Carolus Lassila and his wife were killed in a car accident in the Atlas Mountains of Morocco on their way from Ouarzazate to Taznakht, 250 miles south of Rabat on October 26, 1987, as they drove their car the river suddenly flooded after heavy rain.