Human dental calculus is an excellent target for examining the plant component of ancient diets. Microfossils become imbedded within dental calculus throughout life, providing an overall picture of plant foods available (at least those that produce recoverable microfossils). Here we evaluate previous phytolith results (Dudgeon and Tromp, 2012) by examining starch grains from 30 human dental calculus samples from 10 archaeological sites throughout Rapa Nui (Easter Island), dating between AD 1330–1900. The unobscured starch grains recovered are consistent with descriptions of modern reference samples of sweet potato (Ipomoea batatas). These results indicate the importance of sweet potato to the Rapanui diet prior to European contact in 1722. The analysis of modern sweet potato skins show that they incorporate phytoliths as they grow in phytolith rich sediment, and we argue that the high frequency of palm phytoliths recovered from dental calculus, in conjunction with our starch results points to the consumption of sweet potato.