  The layered quasi-one-dimensional molecular superconductor (TMTSF)$_2$PF$_6$
is a very exotic material with a superconducting order parameter whose ground
state symmetry has remained ill-defined. Here we present a pulsed NMR Knight
shift (K) study of $^{77}$Se measured simultaneously with transport in
pressurized (TMTSF)$_2$PF$_6$. The Knight shift is linearly dependent on the
electron spin susceptibility $\chi_s$, and is therefore a direct measure of the
spin polarization in the superconducting state. For a singlet superconductor,
the spin contribution to the Knight shift, K$_s$, falls rapidly on cooling
through the transition. The present experiments indicate no observable change
in K between the metallic and superconducting states, and thus strongly support
the hypothesis of triplet p-wave superconductivity in (TMTSF)$_2$PF$_6$.
