Isak Gustaf Alfred Collijn (17 July 1875 – 28 March 1949) was a Swedish bibliographer and publisher.
He enrolled as a student in 1893, and graduated with a degree in philosophy in 1902, with the treatise Les suffixes toponymiques dans les langues française et provençale. He was employed as a lecturer at Uppsala University in 1905, and as chief librarian at the same university in 1910. In 1914, Collijn commenced editing the librarians' periodical Nordisk tidsskrift för bok- och biblioteksväsen.
He was a cousin of Ludvig Collijn.