The purpose of this study is to contribute to a better understanding of the complexity of conceptualising and measuring learner engagement in Massive Open Online Courses (MOOCs). The paper develops and validates a MOOC engagement scale (MES) to measure learner engagement. The initial questionnaire items of the scale were developed by reviewing existing related literature and validated student engagement constructs. This was followed by a modified scale development process, including two focus group interviews (n = 10), an exploratory survey (n = 12), an expert review (n = 10), a pilot survey (n = 15), an item purification study (n = 590) and a construct validation study (n = 303). The final version of the scale is made up of four dimensions: behavioural engagement, cognitive engagement, emotional engagement and social engagement. The paper concludes with recommendations for how the MES can be used to assess the pattern of engagement in MOOCs and to investigate the relationship between learner engagement and other important MOOC teaching and learning factors.