Greeniopsis Merr. is one of the poorly known endemic genera of the Philippine Rubiaceae. There is currently little published information on the phytochemistry and biological activities of Greeniopsis species, while important biological activities have been documented for several species of its sister-genus, Ixora L. This study aimed to screen for the presence of major chemical constituents and investigate the antioxidant and antimutagenic potentials of the methanolic leaf extract of Greeniopsis sibuyanensis Elmer. Standard phytochemical screening methods determined the class of plant constituents. The DPPH (2,2-diphenyl-1- picryhydrazyl) radical scavenging and the lipid peroxidation inhibition on -carotene linoleate model system (-CLAMS) assays tested the antioxidant activities of the extract against - Tocopherol. The SOS-red fluorescent protein bioassay system (SOS-RFP) measured the anti-mutagenic activity of the extract. Phytochemical screening confirmed the presence of alkaloids, steroids, terpenoids, flavonoids, saponins, and tannins. Total phenolic content (TPC) and total flavonoid content (TFC) were found to be 20.06 mg gallic acid equivalents and 4.29 mg quercetin equivalents per gram crude extract, respectively. The EC50 values in
the DPPH radical scavenging assay was 42.40 g/mL for -Tocopherol, and 650.72 g/mL for the crude extract. The antioxidant activity based on lipid peroxidation inhibition in the - CLAMS was found to be 76.84% and 22.20% for -Tocopherol (1000 ppm) and crude extract (2500 ppm), respectively. The crude extract was non-mutagenic in the SOS-RFP. It was antimutagenic against mitomycin C-induced damage with an IC50 of 174.84 ppm. The preliminary data on the secondary metabolites of G. sibuyanensis and the observed antioxidant and anti-mutagenic properties may lead to further chemical and biological investigation for its utility to health and wellness.