We present optical photometric and spectroscopic observations of the Type IIb supernova (SN) 2017ati. It reached the maximum light at about 27~d after the explosion and the light curve shows a broad, luminous peak with an absolute r-band magnitude of M_r_=-18.48+/-0.16mag. At about 50d after maximum light, SN 2017ati exhibits a decline rate close to that expected from the ^56^Co --> ^56^Fe radioactive decay, at 0.98mag per 100-days, as usually observed in SNe IIb. However, it remains systematically brighter at late times by about 1-2mag, exceeding the usual upper luminosity range of this class. As a result, modelling the light curve of SN 2017ati with a standard ^56^Ni decay scenario requires a large nickel mass of up to ~0.37M_{sun} and still fails to reproduce the early-time light curve adequately. In contrast, incorporating additional energy input from a magnetar yields a significantly improved fit to the light curve of SN 2017ati, which would reduce the nickel mass to ~0.21M{sun}, still close to the upper end of the range typically inferred for SNe IIb. Comparing the fitted results of SN~2017ati with the known sample of SNe IIb indicates that its luminosity evolution is best explained by a combination of neutron star spin- down energy and radioactive nickel deposition. From late-time nebular spectra of SN 2017ati, the luminosity of the [OI]{lambda}{lambda}6300,6364 doublet implies an oxygen mass of ~1.82-3.34M{sun}, and the combination of a [CaII]/[OI] flux ratio of ~0.5 with nebular spectral model comparisons favours a progenitor zero-age main-sequence mass of >=17M{sun}_.