This book offers a timely and comprehensive exploration of how generative AI technologies are reshaping the landscape of online disinformation, dramatically lowering the barriers to creating realistic synthetic text, imagery, and audio at scale. While online disinformation has long challenged societies and individuals, the acceleration of generative AI has intensified these risks, making traditional detection and protection measures increasingly inadequate. In response, researchers are developing advanced AI-driven tools and methods to detect, mitigate, and study the rapidly evolving tactics of disinformation campaigns, with emphasis on algorithmic detection, media verification, and the broader implications for information integrity. Countering Disinformation in the Era of Generative AI brings together leading experts to present both recent misuse cases -- such as synthetic media in political manipulation and AI-generated false narratives -- and state-of-the-art countermeasures, including trustworthy AI solutions co-created with media professionals. The book features thorough coverage of emerging methods for detecting disinformation and synthetic media and analyzing online information environments to. Drawing upon interdisciplinary research and large-scale initiatives such as the Horizon Europe vera.ai project, this volume addresses the needs of diverse stakeholders -- from researchers and journalists to policymakers -- offering not only technical depth for specialists, but also an accessible overview of the latest advances in AI-powered disinformation defense.