Recently, a few investigations were conducted to understand the electrocatalytic activity of the pyrite FeS$_2$ towards hydrogen evolution reaction (HER) in acidic medium. A systematic investigation to understand its catalytic activity towards both HER and oxygen evolution reaction (OER) in alkaline medium is important, but rare. It was found that iron sulphides have inferior H-adsorption efficiency, which hindered their HER catalytic activity. Herein, a novel strategy was undertaken to obtain pyrites having different crystallite sizes and lattice strains. Changes in these crystal parameters affected the physicochemical phenomena occurring at the electrode–electrolyte interface which in turn influenced the electrocatalytic activity of the FeS$_2$ and also altered the pathways of the reactions. The pyrite with the lowest lattice strain and crystallite size (FS3) showed superior catalytic activity towards both HER and OER. The overall water-splitting activity of FS3 was comparable with the state-of-the-art RuO$_2$–Pt/C couple. Moreover, the synthesized pyrites showed the capability to overcome the previously mentioned drawback i.e., inferior H-adsorption efficiency. The pyrite FeS$_2$ could be a potential candidate as a cheap and efficient bifunctional electrocatalyst for overall water splitting.This investigation demonstrates that modulation of crystal parameters can be an efficient technique to tune the activity ofan electrocatalyst.