Background Postoperative atrial fibrillation is a common consequence of cardiac surgery with increased stroke complications and mortality. Although dexmedetomidine is thought to prevent postoperative atrial fibrillation and stroke because of its sympatholytic and anti-inflammatory properties, data from different studies show the effect of dexmedetomidine on postoperative atrial fibrillation and stroke uncertain in adult patients with cardiac surgery. Methods A database including EMBASE, PubMed, and Cochrane CENTRAL was searched for randomized controlled trials comparing dexmedetomidine with placebo or other anesthetic drugs in adult cardiac surgery. The primary outcome was the incidence of postoperative atrial fibrillation. The secondary outcomes were the incidence of postoperative stroke, mechanical ventilation duration, intensive care unit length of stay, hospital length of stay, and mortality. Results Eighteen trials with a total of 2933 patients were enrolled in the meta-analyses. Compared with controls, dexmedetomidine significantly reduced the incidence of postoperative atrial fibrillation [odds ratio, 0.82; 95% CI, 0.69-0.98; P = .03]. There was no significant difference between groups in stroke (odds ratio, 1.36; 95% CI, 0.59-3.16; P = .47), mechanical ventilation duration [weighted mean difference, −0.17; 95% CI, −0.35 to 0.14; P = .39], intensive care unit length of stay (weighted mean difference, −0.03; 95% CI, −0.93 to 0.87; P = .95), hospital length of stay (weighted mean difference, −0.04; 95% CI, −0.40 to 0.32; P = .83) and mortality (odds ratio, 0.72; 95% CI, 0.32-1.60; P = .42). Conclusion Perioperative dexmedetomidine reduced the incidence of postoperative atrial fibrillation in adult patients undergoing cardiac surgery. But there was no significant difference in the incidence of stroke, mechanical ventilation duration, intensive care unit length of stay, hospital length of stay, and mortality.