The International Atomic Energy Agency (IAEA) programme ‘Modelling and Data for Radiological Impact Assessments’ (MODARIA), which ran from 2012 to 2015 had the general aim of improving capabilities in the field of radiological and environmental dose assessment. Within MODARIA Working Group 3 ‘Application of models for assessing radiological impacts arising from NORM and radioactively contaminated legacy sites to support the management of remediation’, a software tool, NORMALYSA (NORM And LegacY Site Assessment), was developed. NORMALYSA can be used to simulate radionuclide transport in the environment from the source term (e.g. radioactively contaminated land) to the relevant receptors (e.g. residential areas, agricultural areas, water bodies) and to estimate resulting radiation exposure and corresponding doses to humans. This publication is the User Manual for NORMALYSA v.2.3 software, which also contains a description of the overall software architecture and the functionality of NORMALYSA.