  Using data from the Neutron star Interior Composition ExploreR (NICER)
observatory, we identified a permanent spin frequency decrease of
$\Delta\nu=-(1.04\pm0.07)\times 10^{-7}\,\mathrm{Hz}$ around MJD 60132 in the
rotation-powered pulsar PSR B0540-69, which exhibits a periodic signal at a
frequency of $\nu\sim 19.6\,\mathrm{Hz}$. This points to an anti-glitch event,
a sudden decrease of the pulsar's rotational frequency without any major
alteration in the pulse profile or any significant increase of the pulsed flux.
Additionally, no burst activity was observed in association with the
anti-glitch. To date, observations of the few known anti-glitches have been
made in magnetars or accreting pulsars. This is the first anti-glitch detected
in a rotation-powered pulsar. Given its radiatively quiet nature, this
anti-glitch is possibly of internal origin. Therefore, we tentatively frame
this event within a proposed mechanism for anti-glitches where the partial
`evaporation' of the superfluid component leads to an increase of the normal
component's moment of inertia and a decrease of the superfluid one.
