We report the first soft gamma-ray observation by the OSSE experiment on board the Compton Gamma Ray Observatory (CGRO) of the optically selected Seyfert 2 galaxy NGC 4507. The source was observed on two separate weeks in 1993 and detected between 50 and 200 keV at a confidence level of 7 sigma. There is no evidence of flux variability within or between the two observation periods which sample timescales from days to months. The source spectrum obtained from combining the two sets of data is best described by spectral forms including some exponential cutoff. For example, the best-fit thermal bremsstrahlung model provides a temperature of kT = 95(-25)(+31), while a thermal Comptonization model gives a plasma temperature of 44(-9)(+)7 keV for an optical depth of 3; the 100 keV intensity is typically 5 x 10(-6) photons cm(-2) s(-1) keV(-1). The observed shape is steeper than the one observed by Ginga in the X-ray band in 1990. The joint Ginga-OSSE spectrum is best described by an absorbed power law exponentially cut off at high energies and having an iron K alpha emission line; acceptable fits can be obtained without (f(r) = 0) or with (f(r) = 1) a Compton reflection component. In the first case we find a harder photon index (Gamma = 1.3 +/- 0.2) and a lower cutoff energy (E(c) = 73(-24)(+48) keV) than in Seyfert 1 galaxies, while in the second case the fit parameters are within the range of values observed in type 1 objects (Gamma = 1.7 +/- 0.2 and E(c) = 150(-66)(+290) keV); the statistics are, however, insufficient to discriminate between these two spectral shapes, although a slightly better fit is obtained with reflection included. Comparison between NGC 4507 and other Seyfert galaxies indicates that although al low energies a difference between type 1 and type 2 objects is possible (but still weak in this specific case), at high energies the observed shapes are indeed similar. We interpret this as evidence that the primary source emission (i.e., not reprocessed by material in the source) is the same for both types; this finding is in agreement with the unified scheme of Seyfert galaxies. Apart from its own interest, the OSSE detection of hard X-ray emission from a Seyfert 2 galaxy is also important because of the implications it has for the contribution of Seyfert galaxies of both types to the cosmic diffuse background, since type 2 objects are expected to outnumber type 1 sources.
