Reconstructing the history of Holocene relative sea levels around Tonga provides essential constraints on the recent geological evolution of this region and paleoenvironmental context for archaeological studies. However, few sea level records are currently available in the region, and no quantitative paleoenvironmental studies using geochemical or geophysical methods have been reported. Here, we reconstruct the sea level history of Tongatapu Island using radiocarbon measurements and glacio-hydro-isostatic adjustment (GIA) modeling. Our analyses reconstructing the evolution of the lagoon suggest that the average size of Gafrarium tumidum decreased synchronously with corresponding changes in the paleoenvironment. These changes also correspond to the increasing trend of the lagoon specific local marine reservoir ages (?Rlagoon) from 219 ± 78 to 368 ± 69 years between ?2.6 and 0.4 ka. Sea surface salinity (SSS) decline within Fanga 'Uta lagoon was also synchronous with these changes caused by a gradual decrease in the exchange of water in and out of the lagoon. Estimated SSS from shell was somewhat higher ca. 2.6 cal kyr B.P. than the present, suggesting that the lagoon was relatively open to the ocean at that time. Our GIA modeling predicts mid-Holocene sea level highstand (HHS) was less than 1 m above the present sea level in Tongatapu, suggesting that previously reported observations of an HHS require additional contributions, perhaps from crustal uplift. Furthermore, recent GNSS observations of vertical uplift rates at Tongatapu are an order of magnitude higher than the long-term uplift rate obtained from Holocene sea level data.