Sepsis remains a major cause of mortality in intensive care units worldwide, with an estimated 49 million cases and over 11 million deaths annually, highlighting the need for earlier detection to improve outcomes. This systematic review synthesizes evidence on machine learning models for early sepsis prediction in adult ICU patients from 2017 to 2021, focusing on prediction horizons, data modalities, and validation approaches. A comprehensive search of PubMed, Embase, IEEE Xplore, ACM Digital Library, and arXiv identified studies meeting criteria for ICU-based sepsis prediction with at least a 4-hour forecast window, following PRISMA guidelines. Of 1,478 records screened, 35 studies were included, with prediction horizons ranging from 4 to 24 hours and most relying on hourly vital sign data and internal validation. Reported performance varied widely depending on horizon length, data sampling, and validation rigor, with external validation generally producing lower but more realistic results. Overall, while machine learning models show promising predictive ability, limitations in generalizability and standardization remain, emphasizing the need for stronger validation frameworks and reporting practices to support clinical translation.