We have studied the mass currents through a superfluid He-3 Josephson weak
link in the presence of an externally applied ac pressure modulation. Chara
cteristic changes in the dc mass currents are observed whenever the superfl
uid Josephson frequency omega (J) is an integer multiple of the ac modulati
on frequency omega. The measured dependencies of these current changes on a
c pressure amplitude are in excellent agreement with theory describing quan
tum phase dynamics of superfluid He-3 weak links. These results establish t
he superfluid analog of the superconducting Shapiro effect.