Evidence of diastolic dysfunction (DD) required for the diagnosis of heart failure with preserved ejection fraction (HFpEF) is elusive in atrial fibrillation (AF). Left ventricular (LV) and left atrial (LA) speckle-tracking echocardiography (STE) may provide rhythm independent indications of DD. We aimed to find common LV/LA myocardial mechanics parameters to demonstrate DD, using STE in patients with AF. 176 echocardiographic assessments of patients were studied retrospectively by STE. 109 patients with history of AF were divided in three groups: sinus with normal diastolic function (n = 32, ND), sinus with DD (n = 35, DD) and patients with AF during echocardiography (n = 42). These assessments were compared to 67 normal controls. Demographic, clinical, echocardiographic and myocardial mechanic characteristics were obtained. The patients with DD in sinus rhythm and patients with AF were similar in age, mostly women, and had cardiovascular risk factors as well as higher dyspnea prevalence compared to either controls or patients with ND. In the AF group, LV ejection fraction (LVEF) (p = 0.008), global longitudinal strain and LA emptying were lower (p < 0.001), whereas LA volumes were larger (p < 0.001) compared to the other groups. In a multivariable analysis of patients in sinus rhythm, LA minimal volume indexed to body surface area (Vmin-I) was found to be the single significant factor associated with DD (AUC 83%). In all study patients, Vmin-I correlated with dyspnea (AUC 80%) and pulmonary hypertension (AUC 90%). Vmin-I may be used to identify DD and assist in the diagnosis of HFpEF in patients with AF.