Background: Left ventricular diastolic dysfunction is a predictor of adverse outcome after acute myocardial infarction (AMI). We aimed to test if sleep-disordered breathing (SDB) contributes to the development of diastolic dysfunction in patients with preserved left ventricular ejection fraction after AMI. Method: Patients with AMI, percutaneous coronary intervention and an ejection fraction ≥50% were included in this sub-analysis of a prospective observational study. Patients with AMI (n = 41) underwent cardiovascular magnetic resonance imaging (volume–time curve analysis) to define diastolic function by means of the normalised peak filling rate [nPFR; (end diastolic volume/second)]. In patients with AMI, the nPFR was assessed within <5 days and three months after AMI. Patients with AMI were stratified in patients with (apnoea-hypopnoea index, AHI ≥15/h) and without (AHI <15/h) SDB as assessed by polysomnography. Results: At the time of AMI, the nPFR was similar between patients with and without SDB (2.90 ± 0.54 vs. 3.03 ± 1.20, p = 0.662). Within three months after AMI, diastolic function was significantly lower in patients with SDB than in patients without SDB (ΔnPFR: −0.83 ± 0.14 vs. 0.03 ± 0.14; p < 0.001; ANCOVA, adjusted for baseline nPFR). In contrast to central AHI, obstructive AHI was associated with a lower nPFR three months after AMI, after accounting for established risk factors for diastolic dysfunction [multiple linear regression analysis, B (95%CI): −0.036 (−0.063 to −0.009), p = 0.011]. Conclusion: Our data indicate that obstructive sleep apnoea impairs diastolic function early after myocardial infarction.