Science is addressing global societal challenges and, due to limitations in research financing, scientists are turning to public at large to jointly tackle specific environmental issues. Citizens are therefore increasingly involved in monitoring programs, appointed as citizen scientists with potential to delivering key data at near no cost to address environmental challenges, so fostering scientific knowledge and advise policy- and decision-makers. One of the first and most successful example of marine citizen science in the Mediterranean is represented by the integrative and collaborative implementation of several jellyfish spotting campaigns in Italy, Spain, Malta, Tunisia started in 2009. Altogether, in terms of time coverage, geographic extent, and number of citizen records, these represent the most effective marine citizen science campaign so far implemented in the Mediterranean Sea. Here we analyzed a collective database merging records over the above four Countries, featuring more than 100,000 records containing almost 25,000 observations of jellyfish specimens, collected over a period of 3 to 7 years (from 2009 to 2015) by citizen scientists participating in any of the national citizen science programs included in this analysis. Such a wide citizen science exercise demonstrates to be one of the so far available most valuable and cost-effective tools to understanding ecological drivers of jellyfish proliferations over the Western and Central Mediterranean basins, and a powerful contribute to develop tailored adaptation and management strategies, mitigate jellyfish impacts on human activities in coastal zones, and support implementation of marine spatial planning, Blue Growth and conservation strategies.