Fredrik Pacius (Finnish pronunciation: [ˈfreːdrik ˈpɑːsius], Finland Swedish: [ˈfreːdrik ˈpɑːsiʉs] ); in German and in Estonian Friedrich Pacius; 19 March 1809 – 8 January 1891) was a German-Finnish composer and conductor who lived most of his life in Finland. He has been called the "Father of Finnish music".

Pacius was born in Hamburg. He was appointed music teacher at the University of Helsinki in 1834. In Helsinki he founded a musical society, the student choir Akademiska Sångföreningen and an orchestra. In 1848, Pacius wrote the music to the poem "Vårt land" by Johan Ludvig Runeberg, which was to become commonly accepted as Finland's national anthem. The same melody was also used for the Estonian national anthem "Mu isamaa, mu õnn ja rõõm" and the Livonian ethnic anthem "Min izāmō, min sindimō".

In 1852, he composed Kung Karls jakt (English: King Charles' Hunt; Finnish: Kaarle-kuninkaan metsästys), which was the first Finnish opera, with a libretto in the style of Romantic nationalism, like the national anthem designed to convince Finland's grand duke (i.e. the Russian Emperor Nicholas I) of the total loyalty of his subjects in Finland. The libretto was written by the author and historian Zacharias Topelius in close collaboration with Pacius.

His compositions also include a violin concerto, a symphony, a string quartet and several other operas.

Pacius died in Helsinki, aged 81.