This study explores how hydroalcoholic extracts of Campomanesia lineatifolia R. and P. seeds affect the germination and viability of seeds from two weeds, Rumex crispus and Amaranthus hybridus. The phenolic compounds were initially characterized using ultra-high-performance liquid chromatography. In seeds sown in Petri dishes, four concentrations of hydroalcoholic extracts (0%, 3%, 6%, and 9% w/v) were used for single and continuous applications. The mean germination speed, mean germination time, germination percentage, and seed viability were evaluated. Epigallocatechin gallate, quercetin-3-glucoside, epicatechin gallate, ursolic acid, epigallocatechin, and trans-cinnamic acid were the main compounds discovered in that order. Notably, while the germination percentages of both species were reduced with a single application (38.5% for R. crispus and 52% for A. hybridus), they were reduced by 76.2% and 58.34% with a continuous application, respectively. A delay in germination was also observed, which produced changes in germination speed throughout the experiment. With a 9% application, the number of nonviable seeds of R. crispus increased by 40.53%, whereas in A. hybridus, viability decreased by at least 70.8%. Overall, the phenolic compounds in C. lineatifolia extract are thought to inhibit the germination process of the evaluated species.