Phytoestrogens exert pleiotropic effects on cellular signaling and show some beneficial effects on estrogen-dependent diseases. However, due to activation/inhibition of the estrogen receptors ERα or ERβ, these compounds may induce or inhibit estrogen action and, therefore, have the potential to disrupt estrogen signaling. We performed a comprehensive analysis and potency comparison of phytoestrogens and their human metabolites for ER binding, induction/suppression of ERα and ERβ transactivation, and coactivator recruitment in human cells. The soy-derived genistein, coumestrol, and equol displayed a preference for transactivation of ERβ compared to ERα and were 10- to 100-fold less potent than diethylstilbestrol. In contrast, zearalenone was the most potent phytoestrogen tested and activated preferentially ERα. All other phytoestrogens tested, including resveratrol and human metabolites of daidzein and enterolactone, were weak ER agonists. Interestingly, the daidzein metabolites 3′,4′,7-isoflavone and 4′,6,7-isoflavone were superagonists on ERα and ERβ. All phytoestrogens tested showed reduced potencies to activate ERα and ERβ compared to diethylstilbestrol on the estrogen-responsive C3 promoter compared to a consensus estrogen response element indicating a degree of promoter dependency. Zearalenone and resveratrol were antagonistic on both ERα and ERβ at high doses. The phytoestrogens enhanced preferentially recruitment of GRIP1 to ERα similar to 17β-estradiol. In contrast, for ERβ no distinct preference for one coactivator (GRIP1 or SRC-1) was apparent and the overall coactivator association was less pronounced than for ERα. Due to their abundance and (anti)-estrogenic potencies, the soy-derived isoflavones, coumestrol, resveratrol, and zearalenone would appear to have the potential for effectively functioning as endocrine disruptors.