Fredrik Pacius (born Friedrich Pacius) (About this sound pronunciation (help·info); March 19, 1809 – January 8, 1891) was a German 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 Finland's national anthem. Pacius’s music was also used for the Estonian national anthem "Mu isamaa, mu õnn ja rõõm" and the Livonian national anthem Min izāmō, min sindimō.

In 1852 he composed Kung Karls jakt (The Hunt of King Charles) 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 the composer.

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

Pacius died in Helsinki, aged 81.