Despite the prepdominat agent causing severe entero-pathogenic diarrhea in swine, there are no effective therapeutical treatment of porcine epidemic diarrhea virus (PEDV). In this study, we evaluated the antiviral activity of five phlorotannins isolated from Ecklonia cava (E. cava) against PEDV. In vitro antiviral activity was tested using two different assay strategies: (1) blockage of the binding of virus to cells (simultaneous-treatment assay) and (2) inhibition of viral replication (post-treatment assay). In simultaneous-treatment assay, compounds 2–5 except compound 1 exhibited antiviral activities of a 50% inhibitory concentration (IC₅₀) with the ranging from 10.8±1.4 to 22.5±2.2μM against PEDV. Compounds 1–5 were completely blocked binding of viral spike protein to sialic acids at less than 36.6μM concentrations by hemagglutination inhibition. Moreover, compounds 4 and 5 of five phlorotannins inhibited viral replication with IC₅₀ values of 12.2±2.8 and 14.6±1.3μM in the post-treatment assay, respectively. During virus replication steps, compounds 4 and 5 exhibited stronger inhibition of viral RNA and viral protein synthesis in late stages (18 and 24h) than in early stages (6 and 12h). Interestingly, compounds 4 and 5 inhibited both viral entry by hemagglutination inhibition and viral replication by inhibition of viral RNA and viral protein synthesis, but not viral protease. These results suggest that compounds isolated from E. cava have strong antiviral activity against PEDV, inhibiting viral entry and/or viral replication, and may be developed into natural therapeutic drugs against coronavirus infection.