A new aflatoxin, aflatoxin $B_{2b}$ (1), together with six known compounds, were isolated from the marine-derived fungus Aspergillus flavus 092008 endogenous with the mangrove plant Hibiscus tiliaceus (Malvaceae). The structure of 1 was determined by the spectroscopic and chemical methods. Compound 1 exhibited a moderate antimicrobial activity against Escherichia coli, Bacillus subtilis and Enterobacter aerogenes, with MIC values of 22.5, 1.7 and 1.1 ${mu}M$, respectively. Compound 1 also showed a weak cytotoxicity against A549, K562 and L-02 cell lines, with $IC_{50}$ values of 8.1, 2.0 and 4.2 ${mu}M$, respectively. The results showed that hydration and hydrogenation of ${Delta}^8$-double bond significantly reduces the cytotoxicity of aflatoxins, while the esterification at C-8 increases the cytotoxicity.