Tannins and related polyphenols have been reported to produce diverse biological effects that indicate their therapeutic potential, including antioxidative, antibacterial, and antiviral activities. In an exploratory study of natural antioxidants with biological effects, we investigated the tannin constituents of myrtaceous plants reported to be rich in polyphenols. We isolated 16 new compounds from five species of the Melaleuca, Eucalyptus, Syzygium, Pimenta, and Myrtus genera. Among the new polyphenols, nine were isolated from Melaleuca squarrosa and characterized as C-glucosidic ellagitannin monomers and oligomers. The structures of the other new polyphenols from myrtaceous plants and a novel compound obtained from the berries of Pyracantha coccinea (Rosaceae) were also elucidated based on spectroscopic and chemical evidence. The antioxidative activity of isolates in the present study was estimated by measuring 1,1-diphenyl-2-picrylhydrazyl radical (DPPH)-scavenging activity and in the oxygen radical absorbance capacity (ORAC) test. We found that oenothein B, a macrocyclic ellagitannin dimer, is widely distributed in the genus Eucalyptus and produces a significant immunomodulatory effect on human dendritic cells (DCs). In a survey of natural ligands that bind to the aryl hydrocarbon receptor (AhR), nepitrin, a flavonoid glycoside, and other polyphenols were revealed to be promising therapeutic candidates.