The ISOLUME project is a response to the JPI Oceans call on the “Consequences of Changing Marine Lightscapes”. This call identifies the urgent need to understand both the drivers and impacts of changes in marine lightscapes on the marine environment that result from both coastal darkening and marine artificial light at night (ALAN) pollution. Our international consortium, led by the Leibniz Institute for Baltic Sea Research Warnemünde (IOW), includes partners from Germany, Poland, Norway, Ireland, UK and Malta. ISOLUME will assess how marine lightscapes have changed across European sea basins over decadal to centennial timescales, and determine drivers, sources and impacts of these changes at both large and small scales. Our approach combines historical trend analyses of in situ and remotely sensed marine optics with statistical and mechanistic modelling. It covers both large, European sea basin scales and focussed smaller scale regional case studies addressing different aspects of drivers and impacts of changing marine lightscapes. The investigation includes marine, estuarine and freshwater systems, land-ocean connectivity and temperature and salinity gradients. The project uniquely investigates four dimensions of marine lightscapes: intensity, location, timing, and spectra, with a focus on changes in the recent 25 years and projections for 2050. The involvement of external partners from European countries, industry, stakeholders, and society is facilitated through a Science Advisory and Stakeholder Panel supported by a scoping activity and a call for expression of interest. The scientific evidence-based knowledge developed in ISOLUME will advance effective monitoring and management strategies and establish policy frameworks to mitigate changing marine lightscapes.