Björn Kristian Ulvaeus (; born 25 April 1945; credited as Björn Ulvæus) is a Swedish songwriter, producer, a former member of the Swedish musical group ABBA (1972–1982), and co-composer of the musicals Chess, Kristina från Duvemåla, and Mamma Mia!. He co-produced the film Mamma Mia! with fellow ABBA member and close friend Benny Andersson.