Here, we investigated the effect of cold plasma (CP) on the biological activities of phloroglucinol. Phloroglucinol (7.92 and 15.84 mM in methanol) was treated with air dielectric barrier discharge plasma at 250 W. In vitro 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging activity and Ferrous reducing antioxidant power (FRAP) values of phloroglucinol increased in a plasma treatment time-dependent manner. CP treatment of phloroglucinol decreased the lipid oxidation of oil emulsion during storage and increased the antimicrobial activity against Bacillus cereus, Escherichia coli O157:H7, and Staphylococcus aureus. Angiotensin-converting enzyme (ACE) inhibitory activity of phloroglucinol increased and total phenolic content decreased based on CP treatment. CP-induced polymerization of phloroglocinol to phlorotannin derivatives was identified using high performance liquid chromatography with UV detector and electrospray ionization/mass spectrometry (HPLC-UV-ESI/MS) method. Consequently, the polymer structure of phloroglucinol was found in CP-treated phloroglucinol. In addition, CP enhances the biological activity of phloroglucinol and could be applied to bioactive materials in food and related industries.