Born in Trondheim, Bratseth started his career at local club Rosenborg BK. There, he would only be a part-time professional until he left for Germany's SV Werder Bremen in January 1987, for a mere €93.000. He was immediately cast into the starting XI, making his club debut in a 1–5 loss at 1. FC Nürnberg on 21 February after the winter break; he would also win the first of his two Bundesliga championships in his first full season.