Seven new isomalabaricane derivatives, rhabdastins A−G (<b>1</b>−<b>7</b>), and a new monocyclic triterpene glycoside, rhabdastoside A (<b>8</b>), have been isolated from the methanol extract of the sponge <i>Rhabdastrella globostellata</i>, collected at Amami-oshima, Japan. Three of them were isolated as their corresponding methyl esters, rhabdastins A−D (<b>1</b>−<b>3</b>). Their structures were determined on the basis of spectroscopic and X-ray diffraction analyses. The isolated compounds were evaluated for their cytotoxicity against the proliferation of promyelocytic leukemia HL-60 cells. Compounds <b>4</b>, <b>5</b>, <b>7</b>, and <b>11</b>, possessing a cyclopentane side chain, exhibited weak activity, with IC₅₀ values of 21, 29, 44, and 11 μM, respectively, while compounds <b>1</b>, <b>2</b>, and <b>3</b>, with a 2-substituted-propanoate side chain, were inactive at 100 μM. In addition, the mechanism of cytotoxicity of compounds <b>4</b> and <b>5</b> was investigated.