Porphyromonas gingivalis is a major periodontitis pathogen that produces several virulence factors including hemagglutinins. These proteins, which are vital molecules, allow P. gingivalis to uptake iron and heme by attaching, aggregating, and lysing erythrocytes. In this study, we evaluated the inhibitory activity of the aqueous extract of Monechma ciliatum seeds against the hemagglutination activity of P. gingivalis. M. ciliatum is a Sudanese medicinal herb that grows in arid and semi-arid lands of tropical Africa. The water extracted from dry powdered seeds was partitioned using ethyl acetate followed by reversed-phase chromatography, thin-layer chromatography, ESI-MS, and NMR analysis resulting in the isolation of four compounds identified as oleic acid, coumarin, 1,2-dioleoylglycerol, and 1,3-dioleoylglycerol with MICs of 15-100 μg/ml against hemagglutination. We believe that the isolation and purification of these compounds will expand the application of M. ciliatum as a natural therapeutic or preventative agent. PRACTICAL APPLICATIONS: Monechma ciliatum or black mahlab is a famous medicinal plant that grows in some parts of arid and semi-arid areas of tropical Africa including western Sudan. Despite its nutritional and traditional medical applications, no studies have evaluated its anti-hemagglutination activity against periodontal pathogens. In this study, four active compounds (oleic acid, coumarin, 1,2-dioleoylglycerol, and 1,3-dioleoylglycerol) were isolated and identified from an aqueous extract of M. ciliatum seeds. The isolated compounds revealed high levels of inhibitory activity against all hemagglutinin agents secreted by Porphyromonas gingivalis. This evidence of inhibitory activity will encourage the application of M. ciliatum effectively as a functional food or therapeutic agent to prevent periodontal diseases in the early stages.