We present optical and near-infrared photometric and spectroscopic observations of SN 2013ej, in galaxy M74, from 1 to 450 d after the explosion. SN 2013ej is a hydrogen-rich supernova, classified as a Type IIL due to its relatively fast decline following the initial peak. It has a relatively high peak luminosity (absolute magnitude MV = −17.6) but a small 56Ni production of ~0.023 M⊙. Its photospheric evolution is similar to other Type II SNe, with shallow absorption in the Hα profile typical for a Type IIL. During transition to the radioactive decay tail at ~100 d, we find the SN to grow bluer in B − V colour, in contrast to some other Type II supernovae. At late times, the bolometric light curve declined faster than expected from 56Co decay and we observed unusually broad and asymmetric nebular emission lines. Based on comparison of nebular emission lines most sensitive to the progenitor core mass, we find our observations are best matched to synthesized spectral models with a MZAMS = 12–15 M⊙ progenitor. The derived mass range is similar to but not higher than the mass estimated for Type IIP progenitors. This is against the idea that Type IIL are from more massive stars. Observations are consistent with the SN having a progenitor with a relatively low-mass envelope.The authors would like to thank Melissa Graham for scheduling the LCOGT observations. This paper is based on observations collected at the European Organization for Astronomical Research in the Southern hemisphere, Chile as part of PESSTO, (the Public ESO Spectroscopic Survey for Transient Objects Survey) ESO programme ID 188.D-3003. The paper is partially based on observations collected at Copernico and Schmidt telescopes (Asiago, Italy) of the INAF – Osservatorio Astronomico di Padova. Some observations have been obtained also with the 1.22 m telescope + B&C spectrograph operated in Asiago by the Department of Physics and Astronomy of the University of Padova. This paper is partly based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnologıa e Innovacion Productiva (Argentina), and Ministerio da Ciencia, Tecnologia e Inovacao (Brazil). This research was made possible through the use of the APASS, funded by the Robert Martin Ayers Sciences Fund. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Parts of this research were conducted by the Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO), through project number CE110001020. IRS was supported by the ARC Laureate Grant FL0992131. SB, AP, NER and GT are partially supported by the PRIN-INAF 2014 project ‘Transient Universe: unveiling new types of stellar explosions with PESSTO’. SSchulze acknowledges support from CONICYT-Chile FONDECYT 3140534, Basal-CATA PFB-06/2007, and Project IC120009 ‘Millennium Institute of Astrophysics (MAS)’ of Initiative Cientıfica Milenio del Ministerio de Economıa, Fomento y Turismo. This work was partly supported by the European Union FP7 programme through ERC grant number 320360. KM acknowledges support from the STFC through an Ernest Rutherford Fellowship. MS acknowledges support from STFC grant ST/L000679/1 and EU/FP7- ERC grant no. [615929]. AGY is supported by the EU/FP7 via ERC grant no. 307260, the Quantum Universe I- CORE Program by the Israeli Committee for Planning and Budgeting and the Israel Science Foundation (ISF); by Minerva and ISF grants; by the Weizmann-UK making connections programme; and by Kimmel and ARCHES awards.This is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/mnras/stw141