A quantitative method involving high-performance thin-layer chromatography (HPTLC) with automated sample application and UV-absorption scanning densitometry was developed for the determination of diphenhydramine hydrochloride in tablets, gelcaps, and capsules. Separation was performed on high performance silica gel plates containing fluorescent indicator, and the analyte was detected as fluorescence-quenched zones under short-wave UV light. Four different pharmaceutical products containing diphenhydramine hydrochloride as an active ingredient were nalyzed to test the applicability of the new method. Precision was validated by replicate analysis of samples and accuracy by analysis of spiked samples. The percent diphenhydramine hydrochloride in the tested pharmaceutical samples ranged from 86.8% to 111% compared to label values, precision ranged from 1.7% to 1.9% relative standard deviation, and the errors in the two spiking experiments were 0.00% and 0.81% compared to the fortification levels.