We present here discovery of a Mira Variable from MASTER Optical Transient J212444.87+321738.3 toward the Cygnus from long-term optical/near-IR photometric and spectroscopic observations. We estimated the period of the variable to be 465 $\pm$ 30 days using the best-fit optical or Near-IR light curves, and strong wavelength dependent variability amplitudes are spanning as $\Delta$I $\sim$ 4 mag, $\Delta$C (400--900~$nm$) $\sim$ 3.4 mag, $\Delta J$ $\sim$ 2.2 mag, $\Delta H$ $\sim$ 1.9 mag and $\Delta K$ $\sim$ 1.5 mag respectively. Optical/Near-IR spectra shows the molecular features like TiO, VO, CO overtone, water bands and likely presence of Si I absorption at 1.59 $\mu$m , which are signatures of typical cool M-type stars. All of these observed characteristics confirm that the source is a O-rich Mira variable. We also study phase-dependent variations of optical/Near-IR spectral features of the object over the Mira phase, which have similar behaviour of Mira variables. We estimated fundamental parameters e.g. effective temperature, spectral class, distance, luminosity of the source from optical/near-IR photometric and spectroscopic observations.