Purpose <p>The study aims to explore the diagnostic value of the Montreal Cognitive Assessment (MoCA) combined with olfactory tests for identifying mild cognitive impairment (MCI) in older adult patients with type 2 diabetes mellitus (T2DM).</p> Methods <p>This prospective study enrolled older adult T2DM patients from Jiulongpo District People’s Hospital, Chongqing, China, between August 2018 and January 2023. Diagnostic criteria proposed by Petersen served as the gold standard for MCI. The Mini-Mental State Examination (MMSE), MoCA, and olfactory assessments were employed to evaluate cognitive function.</p> Results <p>A total of 192 patients were included, of which 94 (49.0%) were diagnosed with MCI (68.5 ± 5.5&#xa0;years; 31.9% male) and 98 (51.0%) with normal cognitive function (NMCI) (68.4 ± 4.9&#xa0;years; 45.9% male). Compared to the NMCI group, the MCI group showed significant differences in MMSE (25.6 ± 2.5 vs. 28.7 ± 1.3, <i>P</i> &lt; 0.001), MoCA (20.7 ± 2.5 vs. 26.3 ± 2.4, <i>P</i> &lt; 0.001), olfactory (23.9 ± 7.2 vs. 26.4 ± 8.4, <i>P</i> = 0.027), and olfactory impairment degree (<i>P</i> = 0.004). The area under the ROC curve (AUC) for the olfactory test was 0.620 (95%CI: 0.541–0.699), for MMSE 0.838 (95%CI: 0.779–0.897), and for MoCA 0.948 (95%CI: 0.918–0.977). For the combination of MoCA with olfactory tests, the AUC of the parallel test was 0.955 (95%CI: 0.929–0.982).</p> Conclusion <p>The MoCA scale is an effective screening tool for MCI in T2DM patients. Additionally, the combination of MoCA and olfactory testing is superior to MoCA alone in identifying MCI in T2DM patients. This combination approach offers a promising diagnostic tool for MCI in T2DM patients.</p>