Depression affects approximately 280 million people worldwide, yet remains substantially underdiagnosed due to the limitations of clinical assessment tools. This systematic review synthesizes 38 studies (2013--2025) on AI-based depression detection across voice, facial expression, physiological signals, and social media modalities, following PRISMA 2020 guidelines. Multimodal fusion consistently outperforms unimodal approaches, with reported F1 gains of 5-15% on benchmark datasets; however, no reviewed system has undergone prospective clinical validation, and reported accuracy figures should be interpreted as upper bounds on clinical utility. Critical open challenges include cross-cultural... More >