<p>The dyeing process of regenerated cellulose generates large amounts of polluted wastewater and requires chemical dyes, which are disposed of in the environment. To address this issue, we attempted the production of pre-dyed regenerated films and fibers by adding indigo during cellulose dissolution in 1-butyl-3-methylimidazolium chloride/dimethyl sulfoxide. Upon addition of the dye, the cellulose solution became deep blue, and optical microscopy revealed numerous undissolved indigo particles. The apparent viscosity at 110&#xa0;°C was higher than that of the control, suggesting the formation of a new network structure involving indigo, cellulose, and ionic liquid. The dope dyed regenerated cellulose films and fibers appeared deep blue in visual observation; however, optical and scanning electron microscopy revealed differences in dyeing mechanisms, which were also strongly reflected in the colorimetric results. WAXD analysis indicates that indigo addition induces changes in the crystallinity of regenerated cellulose, and suggests that indigo molecules are not randomly distributed in the voids of the amorphous regions, but are preferentially associated with the surfaces of small cellulose crystallites. Indigo in the amorphous regions appeared to preferentially interact with specific hydroxy groups of cellulose rather than being randomly distributed. The strength and modulus of the regenerated cellulose films and fibers decreased with reduced crystallinity, while elongation at break increased. Nevertheless, their overall mechanical performance remained comparable to that of conventional regenerated fibers, indicating the potential of this dope dyeing approach as a practical and environmentally friendly alternative to conventional cellulose dyeing.</p> Graphical Abstract <p></p>