<p>FOXO1 (Forkhead box protein O1), a key transcription factor in the FOXO family, plays a critical role in various physiological processes, including cell cycle arrest, apoptosis, and autophagy in tumors, garnering increasing attention for its impact on tumor biology. While prior studies have highlighted FOXO1’s tumor-suppressive function, emerging evidence underscores its oncogenic potential. FOXO1 activity is modulated by alterations in protein expression and post-translational modifications (PTMs) under both physiological and pathological conditions, with phosphorylation serving as a pivotal regulatory switch that influences its subcellular localization, stability, and transcriptional activity. Unlike previous reviews that broadly addressed FOXO1’s PTMs, this article focuses on the specific phosphorylation-based regulatory mechanisms governing FOXO1. We critically examine how phosphorylation, by dictating FOXO1’s activation status and nuclear-cytoplasmic shuttling, serves as a fundamental molecular switch that directs its functional output. This output—either oncogenic or tumor-suppressive—is not intrinsically encoded by the phosphorylation mark itself but is decisively interpreted within the specific tumor context, as we illustrate across distinct tumor types (e.g., breast cancer, liver cancer, hematologic malignancies). This review outlines the phosphorylation-dependent regulatory networks of FOXO1 across various cancers, explores their potential as clinical biomarkers and therapeutic targets, and proposes targeted interventions aimed at precisely modulating these phosphorylation pathways to disrupt tumor-promoting signals while reinstating tumor-suppressive functions. These strategies aim to overcome the limitations of current therapies and offer innovative approaches for the development of personalized, highly effective anti-cancer treatments.</p>