In this work we present the design, the construction and the measurements of the magnet for a so-called spin-rotator (Wien filter), a beam line device used to rotate the spin direction (and the associated magnetic moment) of muons in a beam used for condensed matter research at the Swiss Muon Source at the Paul Scherrer Institute. The design parameters—ooriginating both from the properties of the preferred particle beam as well as the technological constraints for the high-voltage components generating the necessary electric field—were optimized for device compactness, cost and high beam transmission.