Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the virus that caused the global epidemic of COVID-19. 3C-like protease (3CL^pro), called the main protease (M^pro), cleaves 11 sites in the long-stranded protein synthesized from viral genomic RNA. After the cleavage of the long-stranded protein, proteins such as RNA-dependent RNA polymerase are cut off and activated. Therefore, 3CL^pro is crucial for SARS-CoV-2 replication and is the target of drugs against SARS-CoV-2. Polyphenols and triterpenoids in olives can bind to SARS-CoV-2 3CL^pro in in silico and computational studies. However, not all phytochemicals have been shown to inhibit SARS-CoV-2 3CL^pro. Here, we examined the inhibitory effects of olive-derived phytochemicals on SARS-CoV-2 3CL^pro. Among these phytochemicals, luteolin and oleanolic acid inhibited the 3CL^pro activity by about 40%. The actual enzyme inhibitory action of oleanolic acid has not been examined. Thus, we examined the dose response in the inhibition of 3CL^pro by oleanolic acid. Oleanolic acid significantly inhibited the enzymatic activity of 3CL^pro at concentrations above 50 μM, and it may be a potent molecule that inhibit SARS-CoV-2 replication via 3CL^pro inhibition.