Seaweed–bacteria interactions are critical in marine environments, affecting both seaweed health and ecological balance. This chapter examines the definition and relevance of these interactions, emphasizing their function in nutrient cycling, biofilm development, and disease resistance. We explain the molecular processes underpinning seaweed–bacteria interactions, with an emphasis on adhesion, colonization, chemical signaling, and nutrition transfer. Key processes are thoroughly reviewed, including biofilm development, quorum sensing, nitrogen fixation, and phosphorus solubilization. The chapter also covers the genetic control of these relationships, using transcriptome and proteomic analysis to determine the functions of individual genes and gene clusters. Comparative genomics offers evolutionary insights and functional predictions for seaweed-associated microorganisms. Applications include the development of biofertilizers, biostimulants, and functional food additives, as well as strategies for nutrient removal and heavy metal detoxification. The chapter also addresses the challenges and future perspectives in this field, emphasizing the need for advanced omics technologies, synthetic biology approaches, and interdisciplinary research. Understanding and harnessing these interactions not only supports sustainable biotechnological innovations but also aids in the conservation of marine ecosystems.