<p>A yellow natural dye from wood extract of <i>Arcangelisia flava</i> (L.) Merr. was investigated to dye cotton fabrics. In the preliminary phytochemical screening, the aqueous extract of <i>A. flava</i> stem was found to contain tannins/phenolic compounds, flavonoids, alkaloids, and saponins. Several types of mordants such as alum (Al³⁺), lime (Ca²⁺), and ferrous sulfate (Fe³⁺) were used in the dyeing process using three different techniques: pre-mordanting, meta-mordanting, and post-mordanting. The colorimetric data (<i>L</i>^*, <i>a</i>^*, <i>b</i>^*) and color strength (<i>K/S</i>) were measured for the characterization of the dyeing result. Alum was found to be a good mordant to achieve the best results of a bright and desirable yellow color. In this research, the influence of alum mordant concentration on color parameters and wash fastness rating was also conducted. An increasing trend in the <i>K/S</i> value and wash fastness performance of dyed samples was observed: post-mordanting &gt; meta-mordanting &gt; pre-mordanting. The 20% alum post-mordanted sample shows the highest <i>K/S</i> value of 14.19. The post-mordanting method provides the best washing fastness properties compared to other mordanting methods in the range of 4–5 (color change) and 3 (staining change). The higher the concentration of alum tended to increase the color strength of samples in pre-mordanted samples. However, the variation of alum mordant concentration did not significantly influence the wash fastness of the dyed cotton fabrics.</p>