  Quasi-periodic oscillations (QPO) originating from innermost regions of
blazars can provide unique perspective of some of the burning issues in blazar
studies including disk-jet connection, launch of relativistic jets from the
central engine, and other extreme conditions near the fast rotating
supermassive black holes. However, a number of hurdles associated with
searching QPOs in blazars e.g., red-noise dominance, modest significance of the
detection and periodic modulation lasting for only a couple of cycles, make it
difficult to estimate the true significance of the detection. In this work, we
report a $\sim$ 330-day QPO in the Fermi/LAT observations of the blazar Mrk 501
spanning nearly a decade. To establish consistency of the result, we adopted
multiple approaches to the time series analysis and employed four widely known
methods. Among these, Lomb-Scargle periodogram and weighted wavelet z-transform
represent frequency domain based methods whereas epoch folding and
z-transformed discrete auto-correlation function are time-domain based
analysis. Power spectrum response method was followed to properly account for
the red-noise, largely inherent in blazar light curves. Both local and global
significance of the signal were found to be above 99\% over possible spurious
detection. In the context where not many $\gamma$-ray QPOs have been reported
to last more than 5 cycles, this might be one of the few instances where we
witness a sub-year timescale $\gamma$-ray QPO persisting nearly 7 cycles. A
number of possible scenarios linked with binary supermassive black hole,
relativistic jets, and accretion disks can be invoked to explain the transient
QPO.
