Human alterations of coastal wetlands in Hawai‘i began when Polynesians first colonized the Hawaiian Islands more than 1,000 yr ago. There are contrasting hypotheses and results in studies on impact of anthropogenic forcings on Hawaiian coastal wetland ecosystems. Here we report results of a multibiogeochemical proxy investigation of sedimentary carbon and nitrogen dynamics of a coastal wetland, Kawainui Marsh, on the island of O‘ahu, Hawai‘i. Our results show that humans have impacted Kawainui Marsh in two main ways, upland “indirect” impact and wetland “direct” influence. The former, characterized by decreased δ¹³C and increased δ¹⁵N of sedimentary organic matter (SOM) and elevated concentrations of long-chain n-alkanes, reflects human land-use change in the upland areas of the marsh: cultivation and grazing. The latter, peaking between A.D. 1690 and 1750, is characterized by sharply increased sedimentation rate and mass accumulation rate of SOM, decreased δ¹³C and δ¹⁵N, and elevated C/N and total organic carbon (TOC). These results indicate the direct influence of human impact: the Polynesian transformation and utilization of the ponded (open water) area of the marsh into fishponds. This is the first biogeochemical investigation conducted in the marsh. Our results provide valuable new insights into the history of the pre- and post-European human impact on the marsh.