In this study, by using untargeted LC-MS/MS-based multi-informative molecular networking, 12 briarane diterpenoids were isolated from a closed-system cultured soft coral Briareum stechei. Among them, six unreported compounds were identified and named briastechols A-F (1-6). Notably, briastechols A (1) and B (2) possess a rare 11,14-ether bridge, while briastechols C (3) and D (4) feature a tetrahydrofuran moiety with an ether linkage between C-5 and C-8. Additionally, the absolute configurations of briastechol F (6) and briarenol H (10) were determined for the first time via X-ray crystallographic analysis. Evaluation of bioactivity indicated that the crude extract exerted pronounced anti-inflammatory effects, as reflected in the inhibition of superoxide anion formation and elastase secretion in human neutrophils triggered by N-formyl-methionyl-leucyl-phenylalanine/cytochalasin B (fMLF/CB). Among the isolated compounds, briastechol E (5) showed significant anti-inflammatory potential, with IC50 values of 7.89 ± 0.38 μM and 4.53 ± 0.68 μM against superoxide anion generation and elastase release, respectively. These findings underscore the structural novelty and chemical diversity of metabolites from cultured B. stechei using a closed system, highlighting its potential as a sustainable source of anti-inflammatory agents and further supporting the viability of closed aquaculture systems for marine natural product research.