Calliviminones C–H (1–6), six novel Diels–Alder adducts of a polymethylated phloroglucinol derivative and acyclic monoterpene (myrcene), were isolated from the fruits of Callistemon viminalis. Their structures were elucidated on the basis of extensive analysis of NMR spectroscopic data and calculated electronic circular dichroism spectra. Compounds 1 and 2 were the first examples of a polymethylated phloroglucinol derivative connected with myrcene in a hetero-Diels–Alder manner and 3–6 were carbon-Diels–Alder adducts featuring an unusual core of spiro-[5.5] undecene. Bioactivity scans indicated that 2–6 showed moderate inhibition on nitric oxide production in lipopolysaccharide-induced RAW264.7 macrophages.