A novel phenanthrenoid symmetrical dimer 8,8'-bidehydrojuncusol [1,1',6,6'-tetramethyl-5,5'-divinyl-8,8'-biphenanthrene-2,2',7,7'-tetraol], a related phenanthrenoid monomer, a phenolic chromone, and five flavonoids derivatives have been isolated from the halophyte Juncus acutus L., Juncaceae. The structure of the dimeric phenanthrenoid was determined on the basis of spectroscopic analyses, including 2D NMR spectroscopy, and by spectral correlations. The new dimer and the other isolated compounds bearing four phenolic hydroxy groups showed the significant in vitro antioxidant activity comparable with that of ascorbic acid using 2,2'-azino-bis[3-ethylbenzothiazoline-6-sulphonate] (ABTS) radical cation decolourisation assay. On the basis of the results from an in vitro anti-inflammatory assay using lipopolysaccharide-stimulated RAW264.7 macrophage cells linked with immunoblot analysis, it was found that dimerisation of dehydrojuncusol [1,6-dimethyl-5-vinyl-8-phenanthrene-2,7-diol] molecule nearly nullified its inhibitory effect on the expression of the pro-inflammatory inducible nitric oxide synthase (iNOS) protein.