Swinhoeisterols A (1) and B (2), two novel sterols with an unprecedented 6/6/5/7 ring system, were isolated from the sponge Theonella swinhoei. The structures and absolute configurations were elucidated by spectroscopic analysis, X-ray single-crystal diffraction, modified Mosher's method, and TDDFT/ECD calculations. The cytotoxicity of these compounds toward A549 and MG-63 cells encourages studies on their potential target using an inverse virtual screening approach. The predicted inhibitor of h(p300) was corroborated by an in vitro biological test.