We present new experimental and calculated values of the indirect spin-spin coupling constant (1)J((31)P,(1)H) in the PH3 molecule. The line shape analysis of (1)H and (31)P gas-phase NMR spectra recorded at several densities of PH3, followed by extrapolation of the results to the zero-density limit, gives 176.18(2) Hz as the experimental value at 300 K. The coupled-cluster singles-and-doubles (CCSD) model used in the quantum chemical ab initio calculation gives 187.86 Hz as the nonrelativistic equilibrium geometry value; adding the relativistic and temperature corrections we obtain (1)J((31)P,(1)H) = 177.14 Hz at 300 K. The comparison of this ab initio result with the experimental value shows that proper analysis of nuclear relaxation in the gas phase is essential and leads to very good agreement between theory and experiment.