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Gonioridleylactam (<b>1</b>), a new compound, is a unique dimeric aristolactam isolated from the EtOAc extract of the twigs of <i>Goniothalamus ridleyi</i> King. The structure of gonioridleylactam (<b>1</b>) consists of two different aristolactams linked together with two methylenedioxy bridges at C–3/C–3′ and C–4/C–4′, generating a ten-membered ring of [1,3,6,8]tetraoxecine. A new natural product, gonioridleyindole (3-hydroxymethyl-1-methyl-1<i>H</i>-benz[<i>f</i>]indole-4,9-dione, <b>2</b>), together with eight known compounds (<b>3–10</b>) were also isolated from this plant. Their structures were extensively characterized by spectroscopic methods and comparisons were made with the literature. Compounds <b>1–4</b>, <b>7</b>, and <b>9</b> were evaluated for their <i>α</i>-glucosidase inhibitory activity. Of these, 3,5-demethoxypiperolide (<b>7</b>) displayed the highest <i>α</i>-glucosidase inhibitory activity, with an IC₅₀ value of 1.25 µM.