Objective <p>To comparatively evaluate the effects of mineral trioxide aggregate (MTA) mixed with distilled water (DW), phosphate-buffered saline (PBS), and disodium hydrogen phosphate (DSHP) on the root dentine of teeth affected by diabetes mellitus (DM).</p> Materials and methods <p>Five samples of MTA were mixed with either distilled water (DW), phosphate-buffered saline (PBS), or disodium hydrogen phosphate (DSHP). Then, they were assessed for surface characteristics using high-resolution scanning electron microscopy–energy-dispersive X-ray spectroscopy (HRSEM-EDX), surface microhardness via Vickers indentation, and in vitro bioactivity after 21 days of immersion in simulated body fluid. For the other in vitro tests, 82 single-root human permanent premolars (nondiabetic = 41; diabetic = 41) were decoronated to a standardized length of 12 ± 1&#xa0;mm. Following tubular density evaluation (<i>n</i> = 5) using HRSEM, push-out bond strength (<i>n</i> = 18) and fracture resistance (<i>n</i> = 9) were assessed using a universal testing machine, whereas the cement–dentine interface (<i>n</i> = 9) was analyzed using HRSEM-EDX. The root specimens were then randomly subdivided into subgroups based on the different ionic MTA formulations as follows: subgroup I: MTA + DW, subgroup II: MTA + PBS, and subgroup III: MTA + DSHP. Statistical analysis was conducted using GraphPad Prism 10.4.1, using independent t-tests and two-way analysis of variance with Bonferroni correction for comparisons (<i>P</i> &lt; 0.05).</p> Results <p>MTA + PBS exhibited a uniform matrix with distinct crystalline structures and the highest microhardness (69.2 ± 3.97 VHN), followed by MTA + DSHP and MTA + DW test materials. All formulations promoted apatite formation, with MTA + PBS showing dense, homogenous platelet-like crystals. Tubular density was reported to be higher in diabetic dentine (<i>P</i> &lt; 0.05). MTA + PBS demonstrated superior cement–dentine interface, push-out bond strength, and fracture resistance, followed by MTA + DSHP and MTA + DW in both DM and non-DM root dentine specimens (<i>P</i> &lt; 0.05).</p> Conclusion <p>DM significantly affects the physicochemical and biological properties of root dentine. Among the various test ionic formulations, MTA + PBS exhibited superior surface characteristics, physicochemical, and biological characteristics compared to MTA + DSHP and MTA + DW. Hence, MTA mixed with phosphate ionic vehicles, is clinically recommended for effective endodontic management of diabetic dentine.</p> Clinical trial number <p>Not applicable.</p>