Neopetrothiazide (1), a pentacyclic isoquinoline quinone, was isolated from a Neopetrosia sp. sponge. The structure elucidation was facilitated by utilizing long-range heteronuclear single quantum multiple bond correlation (LR-HSQMBC) and heteronuclear multiple bond correlation (HMBC) nuclear magnetic resonance (NMR) pulse sequences optimized to detect four- and five-bond 1H-13C heteronuclear correlations. These NMR experiments can help assign proton-deficient structural motifs like neopetrothiazide (1), which has 14 contiguous nonprotonated centers (C, N, and S). Neopetrothiazide (1), with an unprecedented thiazide-fused structural scaffold, is the first natural product containing a thiazide moiety.