Abstract Background Type 2 diabetes mellitus (T2DM) represents a significant risk factor for cardiovascular disease, particularly heart failure with preserved ejection fraction (HFpEF). HFpEF predominantly affects elderly individuals and women, and is characterized by dysfunctions associated with metabolic, inflammatory, and oxidative stress pathways. Despite HFpEF being the most prevalent heart failure phenotype in patients with T2DM, its underlying pathophysiological mechanisms remain inadequately elucidated. Objective This study aims to investigate the effects of diabetes mellitus on myocardial inflammation, oxidative stress, and protein quality control (PQC) mechanisms in HFpEF, with particular emphasis on insulin signaling, autophagy, and chaperone-mediated stress responses. Methods We conducted an analysis of left ventricular myocardial tissue from HFpEF patients, both with and without diabetes, employing a range of molecular, biochemical, and functional assays. The passive stiffness of cardiomyocytes (Fpassive) was assessed in demembranated cardiomyocytes before and after implementing treatments aimed at reducing inflammation (IL-6 inhibition), oxidative stress (Mito-TEMPO), and enhancing PQC (HSP27, HSP70). Inflammatory markers (NF-κB, IL-6, TNF-α, ICAM-1, VCAM-1, NLRP3), oxidative stress markers (ROS, GSH/GSSG ratio, lipid peroxidation), and components of signaling pathways (PI3K/AKT/mTOR, AMPK, MAPK, and PKG) were evaluated using western blotting, immunofluorescence, and ELISA techniques. Results Hearts from diabetic HFpEF patients exhibited significantly heightened inflammation, characterized by the upregulation of NF-κB, IL-6, and the NLRP3 inflammasome. This increase in inflammation was accompanied by elevated oxidative stress, diminished nitric oxide (NO) bioavailability, and impaired activation of the NO-sGC-cGMP-PKG signaling pathway. Notably, dysregulation of insulin signaling was observed, as indicated by decreased AKT phosphorylation and impaired autophagy regulation mediated by AMPK and mTOR. Additionally, PQC dysfunction was evidenced by reduced expression levels of HSP27 and HSP70, which correlated with increased cardiomyocyte passive stiffness. Targeted therapeutic interventions effectively reduced Fpassive, with IL-6 inhibition, Mito-TEMPO, and HSP administration leading to improvements in cardiomyocyte mechanical properties. Conclusion The findings of this study elucidate a mechanistic relationship among diabetes, inflammation, oxidative stress, and PQC impairment in the context of HFpEF. Therapeutic strategies that target these dysregulated pathways, including IL-6 inhibition, mitochondrial antioxidants, and chaperone-mediated protection, may enhance myocardial function in HFpEF patients with T2DM. Addressing these molecular dysfunctions could facilitate the development of novel interventions specifically tailored to the diabetic HFpEF population. Graphical abstract