A new photopolarimeter for the simultaneous measurement of all four Stokes parameters of light is described. The light beam, whose state of polarization is to be determined, strikes at oblique incidence three photodetector surfaces in succession, each of which is partially specularly reflecting and each of which generates an electrical signal proportional to the fraction of the radiation that it absorbs. A fourth photodetector captures the remainder of the light. The four outputs thus developed form a 4 x 1 signal vector I, which is linearly related, I = AS, to the input Stokes vector S. Consequently, S is obtained by S = A(-1)I. The 4 x 4 instrument matrix A must be nonsingular, and this requires that the light beam not remain in one plane. For a given arrangement of four detectors, A can be either computed or determined by calibration. The advantages of this polarimeter are enumerated.