Chemical investigation of extracts from the stems and leaves of Alchorneopsis floribunda Müll. Arg., collected in the Amazon region, was performed. The main isolated compounds were triterpenes (α-amyrin, β-amyrin, lupeol, betulin, betulinic acid, uvaol, erythrodiol and oleanolic acid) and phenolic acid derivatives from 4-hydroxybenzoic acid (gallic and protocatechuic acids and isocorilagin). In the germination assays, high inhibitory allelopathic effects of the extracts and isocorilagin were observed and 2,2-diphenyl-1-picrylhydrazyl radical scavenging activity of isocorilagin was higher than those of the standards used (Trolox and butylated hydroxyanisole). This is the first chemical study of the genus Alchorneopsis (Euphorbiaceae).