[en] Using a Compton polarization analyzer the linear polarization of one photon from the three-photon decay of orthopositronium under several geometrical conditions has been measured. For α₁=45⁰, 75⁰, 105⁰ and 120⁰, the ratio of the number of photons Compton-scattered parallel to the plane of the three photons to the number scattered perpendicular to that plane was found to be 1.20±0.03, 1.53±0.03, 1.76±0.06 and 1.86±0.06, respectively. The linear polarization of the photons along the plane of the three photons was determined as 0.15±0.04, 0.33±0.04, 0.49±0.04 and 0.51±0.05, respectively. This result is in agreement with theoretical predictions. (orig.)