Wearable inertial sensors, especially inertial measurement units (IMUs), are increasingly used in swimming performance analysis because they provide objective and practical monitoring in aquatic settings. This systematic review summarised recent progress in their use for swimming performance analysis, focusing on sensor configuration, outcome types, validation methods, and methodological quality. We searched Web of Science, EBSCOhost, IEEE, and PubMed for studies published in English between January 2020 and February 2026. Eligible studies involved competitive swimmers and used wearable inertial sensors during swimming to assess performance or technique outcomes. We extracted study design, participant characteristics, sensor placement, device setup, swimming tasks, outcome measures, and reference systems. Methodological quality was assessed using an adapted COSMIN Risk of Biaschecklist focused on device-relevant measurement properties. A total of 21 studies met the inclusion criteria. Most were conducted in swimming pools and focused on front crawl. The sacrum or lower trunk wasthe mostcommon sensor attachmentsite, particularly for lap segmentation, swimming velocity, and stroke cycle variables. Configurations with multiple sensors were used more often for detailed assessments of stroke timing, coordination, breathing synchronisation, and body rotation. Evidence was strongest for free-swimming outcomes, especially instantaneous velocity, lap time, stroke rate, stroke count, and related stroke cycle measures. Fewer studies addressed starts, turns, and other specific swimming phases. Overall, wearable inertial sensors appear to be capable tools for swimming performance analysis, but further research is needed to improve methodological consistency and strengthen validation across understudied swimming phases.