Microalbuminuria is said to reflect systemic vascular damage and endothelial dysfunction and is an established indicator of cardiovascular morbidity and mortality. Patients with obstructive sleep apnea (OSA)-chronic obstructive pulmonary disease (COPD) overlap syndrome have worse survival than those with OSA or COPD alone. This study evaluated the association between overlap syndrome and microalbuminuria. Data on patients in whom OSA was suspected and who underwent polysomnography between January 2010 and December 2012 were reviewed. Microalbuminuria was defined as an albumin-creatinine ratio between 20 and 299 mg/g in men and between 30 and 299 mg/g in women. Of 740 consecutive patients, 344 were analyzed. Sixty-four were control participants, 248 had OSA only, 4 had COPD only, and 28 had OSA-COPD overlap syndrome. Prevalence of microalbuminuria significantly increased in the order of control, OSA, and overlap syndrome groups (3.1, 12.9, and 32.1%, respectively; P = 0.0006). After adjusting for age and sex, multivariate logistic regression analysis demonstrated a significant association of overlap syndrome with microalbuminuria compared with OSA (odds ratio, 2.61; 95% confidence interval, 1.02-6.38; P = 0.047), but after adjusting for other confounding factors, the difference in the association did not reach significance (odds ratio, 2.54; 95% confidence interval, 0.93-6.72; P = 0.070). Of 63 patients reevaluated after 3 months of continuous positive airway pressure therapy, the logarithm of the albumin-creatinine ratio in 36 patients with good compliance significantly decreased, but there was no difference in patients with poor compliance. OSA-COPD overlap syndrome was more prevalent than OSA alone in patients with microalbuminuria, but the difference might be mediated by conventional risk factors rather than the addition of COPD itself.