ABSTRACT MEGARA (Multi Espectrógrafo en Gran Telescopio Canarias de Alta Resolución para Astronomía) is the optical integral-field and multi-object spectrograph at the Gran Telescopio Canarias. We have created MEGASTAR , an empirical library of stellar spectra obtained using MEGARA at high resolution, $R=20\, 000$ (full width at half-maximum), available in two wavelength ranges: one centred on Hα, from 6420 to 6790 Å; and the other centred on the Ca ii triplet, from 8370 to 8885 Å (HR-R and HR-I volume phase holographic (VPH)-grating configurations). In this work, we use MEGASTAR spectra, a combination of these two short wavelength intervals, to estimate the stellar parameters, namely the effective temperature, surface gravity, and metallicity (and their associated errors), for a sample of 351 MEGASTAR members with spectral types earlier than B2. We applied a χ2-technique by comparing MEGASTAR data with theoretical stellar models. For those stars with stellar parameters derived in the literature, we obtained a good agreement between the published parameters and ours. In addition to the stellar parameters, we also provide several products, such as the rectified spectra, radial velocities, and stellar indices, for this sample of stars. In the near future, we will use MEGASTAR spectra and their derived stellar parameters to compute stellar population evolutionary synthesis models, which will contribute to a better interpretation of star cluster and galactic spectra obtained with MEGARA.