Objectives: This study's objective was to investigate whether or not a methanolic extract of Mallotus japonicas could decrease H+-K+ ATPase activity and neutralise acid. Materials and Methods: We assessed the total phenolic and flavonoid contents of the sample while it was exposed to varying amounts of standard esmoprazole and methanol extract. Results: The proton pump inhibitory activity of the extract from stomach mucosal homogenate was found to be significant (*P<0.05) and on par with the standard. Conclusions: Based on these findings, it may be concluded that the proton pump can be effectively blocked by the methanolic extract.