Purpose: The assessment of lactate threshold (LT) and its relationship to open-water (OW) performance is crucial. This study aimed (1) to analyze LT in world-class OW swimmers, (2) to compare swimming speed at LT (SSLT) and 4 mmol·L−1 of blood lactate concentration ([La−]; SS4), and (3) to examine the relationships between SSLT and swimming performance. Methods: Twenty world-class and elite (11 male, 26.4 [3.0] y; 9 female, 25.8 [3.6] y) OW swimmers voluntarily participated. A total of 46 (29 male and 17 female) intermittent incremental tests (7 × 400 m) conducted in a 50-m pool were analyzed. Seasonal best performances on 400-, 800-, and 1500-m and 10-km OW swimming events were obtained. Results: The SSLT was 1.62 (0.02) (3.8 [1.0] mmol·L−1) and 1.46 (0.04) m·s−1 (3.0 [0.7] mmol·L−1) in males and females, respectively, which corresponded to 97% of the peak speed reached in the tests. There were no differences (P = .148) between SSLT and SS4 in males; however, SSLT was lower (P = .019) than SS4 in females. The SSLT was negatively correlated with swimming performance, with the exception of 10-km OW and 400-m times in males and females, respectively. Conclusions: World-class and elite OW swimmers exhibited a greatly developed aerobic capacity with LT close to their maximum speed. The SS4 could be used as an approximation to SSLT in males but overestimates true aerobic capacity in females. LT is a useful tool for assessing performance, as OW swimmers with higher SSLT showed better swimming performance.