The active antifouling compounds coumarin and its derivatives were isolated from the cinnamon tree Cinnamomum loureiroi. The antifouling activities were determined using representative soft fouling organisms: the seaweed Ulvapertusa and diatom Navicula annexa. The chemical constituents with antifouling activities were identified as coumarin, hydroxylcoumarin, coumaric acid, and cinnamaldehyde by interpreting nuclear magnetic resonance, and high-resolution mass spectroscopy data. These compounds were isolated from 1.09 g of crude Cinnamomum sp. methanol extract, yielding approximately 18.4, 6.3, 9.8, and 14.7 mg of coumarin, hydroxylcoumarin, coumaric acid, and cinnamaldehyde, respectively. The compounds inhibited U. pertusa zoospores with $EC_{50}$ values of $0.13-0.25{mu}g/mL$, and the diatom N. annexa with $EC_{50}$ of $0.21-0.81{mu}g/mL$.