  The baryonic decay $D^+_s\rightarrow p\bar{n}$ is observed, and the
corresponding branching fraction is measured to be
$(1.21\pm0.10\pm0.05)\times10^{-3}$, where the first uncertainty is statistical
and second systematic. The data sample used in this analysis was collected with
the BESIII detector operating at the BEPCII $e^+e^-$ double-ring collider with
a center-of-mass energy of 4.178~GeV and an integrated luminosity of
3.19~fb$^{-1}$. The result confirms the previous measurement by the CLEO
Collaboration and is of greatly improved precision, which may deepen our
understanding of the dynamical enhancement of the W-annihilation topology in
the charmed meson decays.
