To evaluate phasic function and deformation of the left atrium (LA) and right atrium (RA) in subjects with prediabetes and type 2 diabetes mellitus. This cross-sectional study included 50 untreated normotensive subjects with prediabetes, 60 recently diagnosed normotensive diabetic patients and 60 healthy controls of similar sex and age. All the subjects underwent laboratory analyses and complete echocardiographic examination including strain analysis. LA and RA reservoir and conduit function gradually decreased, while booster pump increased, from the healthy controls, throughout the prediabetics, to the diabetics. The strain analysis of atrial phasic function showed more regular pattern of progressive atrial function deterioration than conventional evaluation with total, active and passive atrial function. In the whole study population HbA1c correlated with LA passive emptying fraction (r = −0.38, p < 0.01), LA active emptying fraction (r = 0.36, p < 0.01), LA longitudinal strain during systole (r = −0.35, p < 0.01), RA passive emptying fraction (r = −0.42, p < 0.01), RA active emptying fraction (r = 0.38, p < 0.01), and RA longitudinal strain during systole (r = −0.32, p < 0.01). However, only LA passive emptying fraction (β = −0.32, p < 0.01) and LA longitudinal strain during systole (β = −0.28, p = 0.02) were independently associated with HbA1c among the LA parameters; whereas solely RA passive emptying fraction (β = −0.37, p < 0.01) and RA active emptying fraction (β = 0.31, p = 0.01) were independently associated with HbA1c among the RA parameters. LA and RA phasic functions are significantly impaired in the prediabetics and the diabetics. The parameter of glucose control correlated with LA and RA reservoir, conduit and pump atrial function.