Ocean ecosystems are crucial for life on Earth, providing food, regulating the climate, and supporting biodiversity. However, the ongoing effects of climate change, such as rising ocean temperatures, ocean acidification, and rising sea levels, are significantly altering marine environments. The impact of climate change on ocean ecosystems is an urgent issue with far-reaching consequences for marine life and human societies. This chapter comprehensively examines the causes and mechanisms driving ocean warming, including the role of greenhouse gases and the heat-buffering capacity of oceans. It discusses the physical and chemical changes occurring in the marine environment, such as increased sea surface temperatures, ocean acidification, deoxygenation, and shifts in ocean currents. The chapter also explores the repercussions for marine life, highlighting the vulnerability of plankton, fish species, coral reefs, and marine megafauna. In addition, the decline in ecosystem services, such as fisheries and biodiversity, and the socioeconomic impacts on coastal communities, food security, and global economies are addressed.