Background <p>This study aimed to assess the expression level of long non-coding RNA H19 (lncRNA H19) in peripheral blood mononuclear cells (PBMCs) of postmenopausal patients with type 2 diabetes mellitus (T2DM) with comorbid osteoporosis (T2DM-OP), and to evaluate its diagnostic value and its role in the pathogenesis of T2DM-OP.</p> Methods <p>A total of 150 postmenopausal patients with T2DM were recruited from the First People’s Hospital of Lianyungang City, China. Based on the bone mineral density (BMD) results measured by dual-energy X-ray absorptiometry (DXA), the patients were stratified into the T2DM-OP group (<i>n</i> = 108) and the T2DM group (<i>n</i> = 42). Clinical data and laboratory parameters were collected, and quantitative reverse transcription polymerase chain reaction (qRT-PCR) was used to quantitatively determine the expression of lncRNA H19 in PBMCs.</p> Results <p>When compared with the T2DM group, the expression level of lncRNA H19 in PBMCs of T2DM-OP patients was significantly lower (<i>P</i> &lt; 0.001). The results of bivariate analysis revealed that lncRNA H19 was positively correlated with albumin (ALB, <i>R</i> = 0.311) and BMD (<i>R</i> = 0.696), while negatively correlated with age (<i>R</i> = -0.514), glycated hemoglobin A1c (HbA1c, <i>R</i> = -0.272), fasting blood glucose (FBG, <i>R</i> = -0.275), and transforming growth factor-β1 (TGF-β1, <i>R</i> = -0.392). All of these correlations were statistically significant (<i>R</i> &lt; 0.001). Binary logistic regression analysis identified age, body mass index (BMI), HbA1c, TGF-β1, and lncRNA H19 as independent risk factors for T2DM-OP. Additionally, multiple linear regression analysis demonstrated that age, HbA1c, FBG, TGF-β1, and BMD were key determinants of the expression of lncRNA H19. Receiver operating characteristic (ROC) curve analysis indicated that lncRNA H19 had high diagnostic accuracy for postmenopausal T2DM-OP, with an area under the curve (AUC) of 0.833.</p> Conclusions <p>The expression level of lncRNA H19 in PBMCs of postmenopausal T2DM-OP patients was significantly reduced. This reduction could contribute to the pathogenesis of T2DM-OP through the regulation of modulating glucose metabolism and the lncRNA H19/TGF-β1 signaling pathway. Moreover, its expression level may serve as a highly promising non-invasive biomarker with high diagnostic accuracy for the early detection of postmenopausal T2DM-OP.</p> Trial registration <p>Retrospectively registered (ChiCTR2300074807).</p>