Humans have become the major driving force of change, deeply affecting the Earth system and the biosphere. In marine ecosystems specifically, climate-related environmental changes and anthropogenic pressures (e.g., fishing, the introduction of new species, nutrient load) have altered the structures and functioning of social-ecological systems (SES). These changes have created novel, never encountered before, SES dynamics. Novelty, a natural process of SES dynamics, has accelerated due to human activities. On the one hand, novelty allows SES to adapt to change, including maintaining their functions and resilience. On the other hand, the fast-emerging novelty in the Anthropocene epoch is unpredictable and increases the uncertainty related to management and predicting models. Despite consensus on the need for acknowledging novelty in SES, there is much confusion associated with this concept. This thesis provides a unifying conceptualization of novelty in SES by linking Complex Adaptive Systems theories and ecological novelty concepts. The papers that make up this thesis are an empirical contribution to understanding novelty in marine SES in the past and future. Novelty was measured in multiple social and ecological components of the Baltic Sea SES across different temporal and spatial scales. Although novelty is important for SES adaptation to change, it can be a problem or a solution - depending on its rate, drivers, and scale. There is a need to foster novelty that could enhance SES resilience and sustainability, in order to achieve good environmental status in marine ecosystems and for human wellbeing.