Citizen Science (CS), the voluntary participation of lay people in scientific work, is well-established in the fields of nature conservation and biodiversity monitoring due to its potential to create large environmental datasets. This study aims to understand the familiarity, perceptions and attitudes towards CS of the key environmental actors in Greece. The target group consisted of employees and/or representatives of Environmental Non-Governmental Organisations (ENGOs), scientists and civil servants related to nature conservation. Quantitative data were collected using an electronic questionnaire, 178 fully completed questionnaires and subsequently eight semi-structured interviews with experts were conducted. Descriptive statistics were used to measure the familiarity and attitude of the actors, as well as the obstacles to the development of CS in Greece. We used Cronbach’s test to measure the reliability of the used Likert scale and Kruskal-Wallis non-parametric test to identify significant differences amongst the three groups of actors. Qualitative data were analysed following a Thematic Analysis methodology. The results show that ca. 40% of the key actors are familiar with the terms and CS practice while over 65% with the concept. The general attitude of the actors towards CS is positive although concerns about data quality collected were highlighted. “Lack of cooperation culture”, “Ignorance of the existence of the phenomenon” and “Lack of know-how” emerged as the most important obstacles to CS development in Greece. Although CS is present in Greece, it is not visible enough. The main reasons are that relevant projects employ different terms, are under-represented in the formal literature and include limited, if at all, project dissemination. There are significant differences regarding familiarity and the attitude towards CS between actors, but also similarities concerning the main obstacles. The study sets a baseline which can be employed to improve and further expand Environmental Citizen Science (ECS) in Greece.