The chemical investigation of a red alga Portieria hornemannii enabled the identification of three new halogenated monoterpenes (1–3) along with two previously identified metabolites (4 and 5). Their structures were determined by spectroscopic analysis and also by utilizing single-crystal diffraction analysis and quantum chemical calculation, as well as by comparison with literature data. Further corrections for dichloro and dibromo carbons using the sorted training set (STS) method were established in this study to significantly improve the accuracy in GIAO 13C NMR calculation of compounds 1–3. To discover the potential bioactive metabolites from P. hornemannii, the anti-inflammatory activities of all compounds were examined. Compounds 1 and 3–5 showed significant anti-inflammatory activity to inhibit the production of pro-inflammatory cytokines in the LPS-stimulated mature dendritic cells.
