A new family of 2D materials, Janus MTeSe (M=Nb, Mo, or W) pristine and defective monolayers have been investigated in this work as promising catalysts for hydrogen evolution reaction (HER) based on first-principles calculations. It has been observed that these Janus monolayers are dynamically and thermodynamically stable. Hybrid exchange-correlation functional (HSE06) based electronic structures reveal Janus NbTeSe is a polarized semiconductor with an indirect bandgap of 1.478 eV with excellent optical absorption capability near infra-red region. While MoTeSe and WTeSe monolayers are direct bandgap semiconductors with a suitable bandgap of 1.859 and 1.898 eV. The carrier effective masses and mobilities in MTeSe monolayer are also calculated. Subsequently, the catalytic activity of pristine as well as defective MTeSe for HER has been identified from the reaction coordinate based on the adsorption free energy ((Formula presented.)). It is noticed that the Nb based Janus layer has comparatively weak HER activity than its peers, group VIB transition metals, Mo, W based Janus layer. The Coulomb attraction between the hydrogen and the monolayer decreases with the increase of the inner atomic radius from Nb, Mo to W, which is one of the structural frailties of 2D Janus NbSeTe as an active photocatalyst. We have further analyzed electronic structures and charge density distributions of pristine and defective MTeSe with/without H adatom to unveil the reason of the catalytic inferiority for Nb based Janus layer over W and Mo based systems. This comparative study of Janus MTeSe monolayers with HSE06 would provide a deep understanding of Janus based HER catalyst. © 2020 The Authors. Published by Wiley-VCH GmbH