With the successful demonstration of wastewater-based epidemiology (WBE) approaches in different countries for estimating the consumption of illicit drugs, researchers are convinced of the approach's potential to include other biomarkers of exposures to complement and expand traditional human biomonitoring efforts. To this end, for the first time, the present study tested the potential to monitor plant-based diet trends in two U.S. cities by measuring levels of phytoestrogens (plant-derived human biomarkers) in influent wastewater. We analyzed monthly samples (2015-2016) of raw wastewater from two midwestern U.S. cities using liquid chromatography-tandem mass spectrometry for three phytoestrogens: daidzein, genistein, and enterolactone. Phytoestrogens were detected in 100% (n = 23) of the samples analyzed, and the concentrations (ng/L) in raw wastewater from Cities 1 and 2, respectively, in decreasing abundance were: enterolactone at 931 ± 267 and 1016 ± 439; daidzein at 821 ± 208 and 783 ± 371; and genistein at 253 ± 90 and 174 ± 83. The relative abundance of phytoestrogens detected in wastewater mirrored their previously reported relative abundance in human urine and is in agreement with the typical plant-based diet (rich in lignans) consumed in the United States. Average phytoestrogen consumption estimated using the WBE approach ranged between 94 and 645 μg/d/capita for these compounds, with City 2 exhibiting higher plant-based consumption than City 1. These estimates were in agreement with consumption values calculated from the previously reported urinary concentration of phytoestrogen for the U.S. population. The present study showed that phytoestrogen are ideal WBE biomarkers to study plant-based diet trends in communities, provided that there is no industrial input of phytoestrogens entering the sewershed.