Depression is a major mental disorder that threatens human physical and mental health. Currently representing an important global public health challenge, finding safe and effective prevention and treatment strategies is of great significance. The characteristic components of omega-3 polyunsaturated fatty acids (ω-3 PUFAs) include alpha linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), which have received continuous attention from the academic community due to their potential antidepressant effects. This study systematically reviews the underlying biological mechanisms of depression, summarizing and analyzing the mode of action of ω-3 PUFAs in preventing and alleviating depression via anti-inflammatory activity, neuroplasticity regulation, and neurotransmitter regulation. Some of the current research limitations and possible future research directions are also addressed. In addition, the paper summarizes the current application status of ω-3 PUFAs in functional foods, dietary supplements, and other nutritional fields. This review aims to provide a comprehensive theoretical reference for the in-depth research, new product development, and clinical applications of ω-3 PUFAs as antidepressant treatment.