Objectives: Antiadherent and antibacterial effects of certain plant extracts have been proven to be beneficial in preventive dentistry. In the present in situ/in vitro crossover study, the impact of plant extracts rich in polyphenols on the erosion-protective properties of the in situ pellicle was evaluated. Methods: Individual splints were prepared for 12 subjects for intraoral exposure of bovine enamel specimens. Following formation of a 1-min pellicle, watery plant extracts (leaves of the wild form of Ribes nigrum, the wild form of Origanum as well as a combination of both) were administered for 10 min in situ. Alternatively, a mouth rinse with fluorides (Elmex Kariesschutz) was performed for 1 min. After further oral exposure for 19/28 min, respectively, slabs were removed and incubated with HCl in vitro over 120 s (pH 2, 2.3, 3). The resulting calcium and phosphate release was quantified photometrically. Slabs with and without a 30-min in situ pellicle served as controls. The modification of pellicle ultrastructure was evaluated by transmission electron microscopy (TEM). Results: Plant extracts modulated the erosion-protective properties of the native in situ pellicle in all test groups in a pH-dependent manner. The combination of R. nigrum leaves and Origanum enhanced the protective properties of the pellicle at all pH values; the administration of this preparation was comparable, yet superior, to the effect of the fluoridated mouth rinse. TEM images indicated that rinsing with R. nigrum leaves/Origanum yielded a distinctly thicker and more electron-dense pellicle. Conclusion: The combination of certain plant extracts offers a novel approach to the complementary prevention of dental erosion.