We have conducted a long-term , wide-field , high-cadence photometric monitoring survey of \sim 50,000 stars in the Lagoon Nebula H ii region . This first paper presents rotation periods for 290 low-mass stars in NGC 6530 , the young cluster illuminating the nebula , and for which we assemble a catalog of infrared and spectroscopic disk indicators , estimated masses and ages , and X-ray luminosities . The distribution of rotation periods we measure is broadly uniform for 0.5 < P < 10 d ; the short-period cutoff corresponds to breakup . We observe no obvious bimodality in the period distribution , but we do find that stars with disk signatures rotate more slowly on average . The stars ’ X-ray luminosities are roughly flat with rotation period , at the saturation level ( \log L _ { X } / L _ { bol } \approx - 3.3 ) . However , we find a significant positive correlation between L _ { X } / L _ { bol } and co-rotation radius , suggesting that the observed X-ray luminosities are regulated by centrifugal stripping of the stellar coronae . The period–mass relationship in NGC 6530 is broadly similar to that of the Orion Nebula Cluster ( ONC ) , but the slope of the relationship among the slowest rotators differs from that in the ONC and other young clusters . We show that the slope of the period–mass relationship for the slowest rotators can be used as a proxy for the age of a young cluster , and we argue that NGC 6530 may be slightly younger than the ONC , making it a particularly important touchstone for models of angular momentum evolution in young , low-mass stars .