Nine novel highly oxygenated and structurally diverse diterpenes, named norstaminolactone A (1), norstaminols B and C (2 and 3), secoorthosiphols A–C (4–6) and orthosiphols R–T (7–9) have been isolated from the aerial part of Orthosiphon stamineus cultivated in Okinawa Prefecture, Japan. Norstaminolactone A (1) is the first representative of a biogenetically unusual norstaminane-type diterpene bearing a nitrogen atom. Norstaminol C (3) possessed a framework presumed to be biosynthesized from the staminane-type diterpene. Secoorthosiphols A-C (4-6) possessed an unprecedented structural feature of the opened ring A system, encountered for the first time in isopimarane-type diterpenes. Secoorothosiphol C (6) also represents the first example of biogenetically unique and unconventional secoisopimarane-type diterpene bearing a cyano group. Norstaminolactone A (1) showed a potent antiproliferative activity with an IC50 value of 2.16 µg/mL against highly liver metastatic colon 26-L5 carcinoma cell line.