Sterculia quadrifida has been empirically used as a traditional plant for medicinal purposes. The objective of this research is to isolate secondary metabolites present in the seeds of S. quadrifida. Extracts of S. quadrifida seeds were prepared using 80% methanol, and then separated into various fractions using liquid-liquid partition with solvents such as n-hexane, chloroform, ethyl acetate, n-butanol, and an insoluble n-butanol fraction. Preparative HPLC was used to purify the compounds, and their structures were identified through spectroscopic analyses. One phenylpropanoid compound, (2E,4E)-1,5-diphenylpenta-2,4-dien-1-one, was isolated from the chloroform fraction and tested for in vitro cytotoxicity using the MTT assay. The compound demonstrated significant cytotoxic effects, with IC50 values of 2.29, 9.93, 18.09, and 12.12 μg/mL in 4T1, MCF-7, MDA-MB-435, and T47D breast cancer cell lines, respectively. Therefore, the phenylpropanoid compound isolated from S. quadrifida has potential as a cytotoxic agent.