In this work we present a method to determine the Raman coefficients of tetrahedrally bonded semiconductors directly in density-functional theory. For this purpose we apply the 2n+1 theorem to derive an analytical expression for the Raman tensor. To the best of our knowledge, this is the first ab initio application of the 2n+1 theorem involving mixed third-order derivatives of the total energy in terms of phonon displacements and electric fields. Numerical results are given for Si, Ge, and various III-V compounds. Furthermore we compare our results with those obtained by frozen-phonon-like calculations and experimental data where available. Our approach can be easily extended to the calculation of Raman tensors for larger systems with other symmetries.