Jag tror på sommaren is a summertime song written by Stig Olin, and recorded by and released as a single in 1967. The Mats Olin recording became a Svensktoppen hit for 12 weeks between 21 May-6 August 1967. The song was written for the final broadcasting of the 1966 autumn season, on 21 December 1966 on request by producer , and performed by Stig Olin together with his wife Britta Holmberg and the children and Lena. Mats performed the song in 1967 at Hylands hörna.