Two new diterpenes, (1E,2R*,3R*,4S*,6E,18S*)-4,18-dihydroxydictyolactone (1) and 8 alpha,11-dihydroxypachydictyol A (2), together with fucoxanthin (3) and a known diterpene 4-hydroxycrenulatane (4) were isolated from the methanol extract of the brown alga Dictyota sp. collected from Bangsaen Beach, Thailand. Their structures were determined by spectroscopic data. The relative stereochemistry of 1 was established by NOE difference and NOESY experiments. Compound 1 showed weak anti-tuberculosis activity while 2 displayed strong cytotoxicity against the NCI-H187 cell line, and potent anti-malarial activity. In addition, 3 was active against HSV-1 and malarial parasites.