Sida spinosa Linn. (synonyms: S. alba L., S. angustifolia Lam., S. angustifolia Mill.), Malvaceae, is an annual erect, branched small perennial herb, which grows abundantly on cultivated fields, waste areas, road sides, and hotter parts of India. S. spinosa has been used in traditional medicine for a variety of therapeutic purposes as astringent, cooling stomachic and in nervous, urinary, and cardiac diseases. A high-performance thin-layer chromatography (HPTLC) method has therefore been established for quantification of kaempferol and apigenin in plant powder of S. spinosa Linn. Chromatographic separation was performed on silica gel plates with dichloromethane-methanol-formic acid 8.0:1.0:0.5 (v/v), as mobile phase. Detection and quantitation were performed by densitometry, with a deuterium lamp, at 340 nm. The responses to reference standards were linear in the concentration range 150–450 μg mL−1 for kaempferol and 50–150 μg mL−1 for apigenin. The relative standard deviation for instrumental precision, intra-assay precision, and intermediate precision was <2%. Kaempferol was present at high concentration, and apigenin was present at low concentration in the whole plant powder. The accuracy of the method was determined by measurement of recovery at three different concentrations. Average recovery was 99.47% for kaempferol and 99.26% for apigenin.The proposed HPTLC method was found to be simple, precise, selective, sensitive, and accurate for routine quality control of whole-plant powder of S. spinosa Linn.