Herman Ernst Henrik Gesellius (16 January 1874 in Helsinki – 24 March 1916 in Kirkkonummi) was a Finnish architect.
Gesellius graduated from the Helsinki University of Technology in 1897. In 1896 he founded the architecture firm Gesellius, Lindgren, and Saarinen, with Armas Lindgren and Eliel Saarinen.
The most famous work projected under his own name is the Wuorio House (Wuorion talo) in Union Street (Unioninkatu), Helsinki. Gesellius designed it from 1908 to 1909, and Lindgren completed it from 1913 to 1914. It features architectural sculpture by Felix Nylund (1878–1940).
Gesllius withdrew from architectural work in 1912 because of serious illness. He died in 1916 of throat cancer.Thalberg House in Helsinki (1897–1898)
Finnish Pavilion at Exposition Universelle in 1900, Paris
"Pohjola" insurance building in Helsinki (1899–1901)
House of Physicians, now Agronomitalo, in Helsinki (1900–1901)
Hvitträsk, home-atelier of architects in Kirkkonummi (1901–1903)
National Museum of Finland in Helsinki (1902–1904)
Club house for the Luther factory in Tallinn (1904–1905)
Vyborg railway station (1904–1913)