The parish of Rennesø was established as a municipality on 1 January 1838 (see formannskapsdistrikt). Mosterøy was separated from Rennesøy on 1 July 1884. It was, however, merged back with Rennesøy on 1 January 1965 (but without Kvitsøy).
The coat-of-arms is from modern times. They were granted on 20 February 1981. The arms show two crosiers. In the present municipality one of Norway's oldest and best preserved monasteries, Utstein, is situated. Hence the choice for the crosier.