We report the isolation, structure elucidation, and antimicrobial activities of two new thiomarinol derivatives, thiomarinols B and C, produced by the marine bacterium Alteromonas rava sp. nov. SANK73390. Thiomarinols B and C were isolated from ethyl acetate extracts of the bacterial culture broth via silica gel column chromatography (to remove thiomarinol A) followed by preparative high-performance liquid chromatography. The structure of thiomarinol B, which contains two additional oxygen atoms compared to thiomarinol A, was determined using nuclear magnetic resonance (NMR), high-resolution fast atom bombardment mass spectrometry (HRFAB-MS), and X-ray crystallography, revealing it as an oxidized derivative with a sulfoxide/sulfone structure in the holothin moiety. Thiomarinol C, with one fewer oxygen atom than thiomarinol A, was identified as 4-deoxythiomarinol A through NMR analysis, acetylation experiments, and hydrolysis studies that confirmed the presence of monic acid C. Thiomarinol B exhibited excellent in vitro antimicrobial activity against Gram-positive and Gram-negative bacteria, including methicillin-resistant Staphylococcus aureus (MRSA), while thiomarinol C showed slightly lower activity than thiomarinol A.