A new arylnaphthalide lignan, prostalidin D (6) was isolated as a minor metabolite from Justicia diffusa var. prostrata. 6 contains a rare catechol unit and its structure was confirmed by total synthesis. In addition, seven known lignans, namely, justicidin E (7), helioxanthin (8), justicidin A (9), medioresinol dimethyl ether (10), medioresinol (11), lariciresinol (12) and 8-methoxyisolariciresinol (13) have also been isolated from the same source and identified by direct comparison with authentic samples.