Left ventricular (LV) diastolic dysfunction and longitudinal systolic dysfunction were identified in patients with diabetes mellitus (DM). This study’s aim was to investigate the impact of LV longitudinal systolic function on LV diastolic function in DM patients with preserved LV ejection fraction (LVEF). We studied 177 DM patients with preserved LVEF (all ≥50%), and 82 age-, gender- and LVEF-matched healthy volunteers as control. Global longitudinal strain (GLS) was defined as the average peak strain of 18 segments from standard apical views, GLS <18% as subclinical LV systolic dysfunction (LVSD), and LV dispersion as the standard deviation of time-to-peak strain from the same views. For DM patients with LVSD (n = 74), E/A and E’ were lower, and E/E’ and isovolumic relaxation time (IVRT) were greater than for DM patients without LVSD (n = 103) and normal controls (n = 82). Moreover, these parameters were lower for DM patients without LVSD than for normal controls. Multivariate analysis revealed that GLS was a strong determinative factor for E’ and E/E’ (β = 0.30, p < 0.001 and β = −0.25, p < 0.001, respectively), as was LV dispersion for E-wave deceleration time and IVRT (β = 0.21, p = 0.002 and β = 0.30, p < 0.001, respectively) independently of age. For normal subjects, however, only age was associated with all LV diastolic parameters. In conclusions, in contrast to age-related LV diastolic dysfunction in normal subjects, in DM patients with preserved LVEF, LV diastolic function was associated with LV longitudinal systolic function and LV dispersion independently of age. Our findings have obvious clinical implications for the management of DM patients.