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Steroid 5α-reductase plays a crucial role in catalyzing the conversion of testosterone to dihydrotestosterone, which is involved in many androgen-dependent disorders. Leaf-hexane extract from <i>Tectona grandis</i> L.f. has shown promise as a 5α-reductase inhibitor. The objectives of this current study were to isolate and identify 5α-reductase inhibitors from <i>T. grandis</i> leaves and to use them as the bioactive markers for standardization of the extract. Three terpenoid compounds, (+)-eperua-8,13-dien-15-oic acid (<b>1</b>), (+)-eperua-7,13-dien-15-oic acid (<b>2</b>), and lupeol (<b>3</b>), were isolated and evaluated for 5α-reductase inhibitory activity. Compounds <b>1</b> and <b>2</b> exhibited potent 5α-reductase inhibitory activity, while <b>3</b> showed weak inhibitory activity. An HPLC method for the quantitative determination of the two potent inhibitors (<b>1</b> and <b>2</b>), applicable for quality control of <i>T. grandis</i> leaf extracts, was also developed. The ethanolic extract showed a significantly higher content of <b>1</b> and <b>2</b> than found in the hexane extract, suggesting that ethanol is a preferable extraction solvent. This study is the first reported isolation of 5α-reductase inhibitors (<b>1</b> and <b>2</b>) from <i>T. grandis</i> leaves. The extraction and quality control methods that are safe and useful for further development of <i>T. grandis</i> leaf extract as an active ingredient for hair loss treatment products are also reported.