Abstract Objective To study the profile of type-2 diabetes (T2D) in patients with RA or OA. Methods This observational, multicentre, cross-sectional study included, over a 24-month period, consecutive patients with adult-onset diabetes and RA or OA. We collected demographics, disease activity and severity indices, current treatments for RA and diabetes, history and complications of diabetes. A systematic blood test was performed, assessing inflammatory, immunological and metabolic parameters. The homoeostasis model assessment (HOMA)2-S was used to assess insulin resistance. Results We included 167 patients with T2D, 118 with RA and 49 with OA. RA and OA patients had severe T2D with suboptimal metabolic control and a biological profile of insulin resistance. Insulin resistance was significantly higher in RA than in OA patients after stratification on age, BMI and CS use [HOMA2-S: 63.5 (35.6) vs 98.4 (69.2), P < 0.001]. HOMA2-S was independently associated with DAS28 [odds ratio (OR): 4.46, 95% CI: 1.17, 17.08]. T2D metabolic control was not related to disease activity and functional impairment, but HbA1c levels were independently associated with bone erosions (OR: 4.43, 95% CI: 1.18, 16.61). Treatment with low-dose CSs was not associated with decreased insulin sensitivity or increased HbA1c levels. Treatment with TNF-α inhibitors was associated with increased insulin sensitivity compared with patients not receiving biologics [101.3 (58.71) vs 60.0 (32.5), P = 0.001]. Conclusion RA patients display severe T2D with inflammation-associated insulin resistance. These findings may have therapeutic implications, with the potential targeting of insulin resistance through the treatment of joint and systemic inflammation.