The industrial use of Eucalyptus urophylla for pulp production generates significant amounts of bark and leaf residues. To support a circular economy, these residues can be utilized as a natural source of tannins for color binding in fabric dyeing via the ecoprinting technique. Compared with batik, a traditional fabric-dyeing method, ecoprinting is considered more environmentally friendly because of its minimal chemical usage, reliance on natural colorants, and absence of wax in the production process. This study aimed to compare the potential of E. urophylla bark and leaves as sustainable sources of tannins for biomordant applications in ecoprinting. Tannins were selected because of their strong affinity for fibers, which enhances color fastness while reducing the reliance on synthetic mordants. An example of the application of tannin, which was obtained from a commercial pulp-production company in North Sumatra, Indonesia, derived from the extraction of bark and leaves of E. urophylla for binding the natural dye derived from Secang wood (Caesalpinia sappan). Ecoprinting was performed on the fabrics via three types of leaves—Swietenia mahagoni, Johar (Cassia siamea), and Krei Payung (Filicium decipiens)—followed by an evaluation of color retention, leaching resistance, and color difference (ΔE) using a colorimeter. The tannin yield from E. urophylla bark was slightly greater than that from the leaves. Both parts contain tannins, along with other phytochemicals, such as alkaloids, flavonoids, terpenoids, steroids, and saponins, in varying amounts. The use of tannins as a biomordant resulted in similar color retention values. Leaching tests indicated that tannins from the bark had a low ΔE at all concentrations, comparable to those of aluminum/calcium mordants, and enhanced the base color. Leaf-derived tannins exhibited very low to low ΔE values, demonstrating greater durability in preventing color leaching. Compared with conventional metal-based materials, tannins provide a viable natural alternative with comparable color stability and eco-friendliness. Ecoprinting applications demonstrated that different leaf combinations influenced the color intensity, with C. siamea producing the strongest color when bark tannins were used and S. mahagoni producing the strongest color when leaf tannins were used. In addition, the use of tannins did not significantly alter the texture of the fabric, thereby ensuring its suitability for textile applications. In addition to its technical benefits, this approach offers environmental advantages such as reduced chemical waste, water savings, and lower carbon emissions than synthetic mordants do. Economically, it can support small-scale enterprises in eco-friendly textile production, foster sustainable economic growth, and contribute to the circular economy, particularly in communities surrounding Eucalyptus plantations. While these findings highlight the potential of E. urophylla tannins in natural dyeing, further research is needed to optimize their extraction efficiency, explore their application to different fabric types, and assess their long-term color stability under real-world conditions. In conclusion, tannins from E. urophylla bark and leaves are promising biomordants for sustainable ecoprinting, offering an eco-friendly alternative to synthetic mordants while strengthening circular economy initiatives in the textile industry.