| Summary: | Four new phenanthrene derivatives, gastrobellinols A-D (<b>1–4</b>), were isolated from the methanolic extract of <i>Gastrochilus bellinus</i> (Rchb.f.) Kuntze, along with eleven known phenolic compounds including agrostophyllin (<b>5</b>), agrostophyllidin (<b>6</b>), coniferyl aldehyde (<b>7</b>), 4-hydroxybenzaldehyde (<b>8</b>), agrostophyllone (<b>9</b>), gigantol (<b>10</b>), 4-(methoxylmethyl)phenol (<b>11</b>), syringaldehyde (<b>12</b>), 1-(4′-hydroxybenzyl)-imbricartin (<b>13</b>), 6-methoxycoelonin (<b>14</b>), and imbricatin (<b>15</b>). Their structures were determined by spectroscopic methods. Each isolate was evaluated for α-glucosidase inhibitory activity. Compounds <b>1</b>, <b>2</b>, <b>3</b>, <b>7</b>, <b>9</b>, <b>13</b>, and <b>15</b> showed higher activity than the drug acarbose. Gastrobellinol C (<b>3</b>) exhibited the strongest <i>α</i>-glucosidase inhibition with an IC<sub>50</sub> value of 45.92 μM. A kinetic study of <b>3</b> showed competitive inhibition on the <i>α</i>-glucosidase enzyme. This is the first report on the phytochemical constituents and <i>α</i>-glucosidase inhibitory activity of <i>G. bellinus.</i>
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