Background <p>This study evaluated the fit and thickness of aligners at tooth embrasures, comparing three-dimensional (3D)-printed and thermoformed aligners.</p> Materials and methods <p>Thirty aligners—3D-printed (no-offset and 0.05&#xa0;mm offset) and thermoformed—were fabricated and mounted on a maxillary dental model. All aligners were scanned using microcomputed tomography. Embrasure gap distance and thickness were measured at coronal and cervical levels, and ratios reflecting aligner fit and thickness changes after fabrication were calculated. The Kruskal–Wallis and Mann–Whitney U tests were used for group and vertical level comparisons.</p> Results <p>The 0.05&#xa0;mm offset 3D-printed aligners exhibited significantly smaller embrasure gaps than the no-offset 3D-printed and thermoformed aligners, with no significant differences between coronal and cervical levels. The other groups showed larger gaps at the cervical level. The embrasure fit ratio was higher in the 0.05&#xa0;mm offset 3D-printed group (73–93%) compared with the no-offset 3D-printed group (58–86%) and thermoformed group (44–85%). Thermoformed aligners demonstrated significant thickness reduction after fabrication, with ratios of 36.5–88.5% relative to the original sheet, whereas 3D-printed aligners exhibited significant thickness increases, with ratios of 197–470% relative to the virtual design.</p> Conclusions <p>The 0.05&#xa0;mm offset 3D-printed aligners exhibited a superior fit compared with the no-offset 3D-printed and thermoformed aligners. The 3D-printed aligners showed significant thickness increases at embrasures, in contrast to the notable thickness reductions observed in thermoformed aligners.</p>