  We report the results of first ever spectro-polarimetric analyses of the
Galactic ultra-luminous X-ray pulsar Swift J0243.6$+6124$ during the 2023
outburst using quasi-simultaneous {\it IXPE}, {\it NICER} and {\it NuSTAR}
observations. A pulsation of period $\sim 9.79$ s is detected in {\it IXPE} and
{\it NuSTAR} observations with pulse fractions (PFs) $\sim 18\%$ ($2-8$ keV)
and $\sim 28\%$ ($3-78$ keV), respectively. Energy dependent study of the pulse
profiles with {\it NuSTAR} indicates an increase in PF from $\sim 27\%$ ($3-10$
keV) to $\sim 50\%$ ($40-78$ keV). Further, epoch-dependent polarimetric
measurements during the decay phase of the outburst confirm the detection of
significant polarization, with the polarization degree (PD) and polarization
angle (PA) ranging between $\sim 2 -3.1\%$ and $\sim8.6^{\circ}-10.8^{\circ}$,
respectively, in the $2-8$ keV energy range. We also observe that the PD
increases up to $\sim 4.8\%$ at higher energies ($\gtrsim 5$ keV) with
dominating \texttt{bbodyrad} flux contribution ($1.5 \lesssim F_{\rm BB}/F_{\rm
PL} \lesssim 3.4$) in the {\it IXPE} spectra. The phase-resolved polarimetric
study yields PD as $\sim 1.7-3.1\%$ suggesting a marginal correlation with the
pulse profiles. Moreover, the broad-band ($0.6-70$ keV) energy spectrum of
combined {\it NICER} and {\it NuSTAR} observations is well described by the
combination of \texttt{bbodyrad} and \texttt{cutoffpl} components with seed
photon temperature ($kT_{\rm bb}$) $\sim 0.86 \pm 0.03$ keV and photon index
($\Gamma$) $\sim 0.98 \pm 0.01$. With the above findings, we infer that the
observed `low' PD in Swift J0243.6$+6124$ is attributed possibly due to `vacuum
resonance' effect between the overheated and relatively cooler regions of the
neutron star boundary layer.
