We report on the design and performance of a highly sensitive combined 3 He/Cs magnetometer for the absolute measurement of magnetic fields. The magnetometer relies on the magnetometric detection of the free spin precession of nuclear spin polarized 3 He gas by optically pumped cesium magnetometers. We plan to deploy this type of combined magnetometer in an experiment searching for a permanent electric dipole moment of ultracold neutrons at the Paul-Scherrer Institute (Switzerland). A prototype magnetometer was built at the University of Fribourg (Switzerland) and tested at Physikalisch-Technische Bundesanstalt (Berlin, Germany). We demonstrate that the combined magnetometer allows Cramér-Rao-limited field determinations with recording times in the range of, measurements above being limited by the stability of the applied magnetic field. With a recording time we were able to perform an absolute measurement of a magnetic field of ≈ with a standard uncertainty of, corresponding to Δ B / B < 6 ×10 −8 . Graphical abstract