Objective: To determine whether frequent premature atrial contractions (PAC) predict atrial fibrillation (AF) in cryptogenic stroke patients, we analyzed the association between frequent PACs in 24-h Holter electrocardiogram recording and AF detected by insertable cardiac monitoring (ICM). Methods: We retrospectively analyzed a database of 66 consecutive patients with cryptogenic stroke who received ICM implantation between October 2016 and March 2018 at 5 stroke centers. We included the follow-up data until June 2018 in this analysis. We defined frequent PACs as the upper quartile of the 66 patients. We analyzed the association of frequent PACs with AF detected by ICM. Results: Frequent PACs were defined as >222 PACs per a 24-h period. The proportion of patients with newly detected AF by ICM was higher in patients with frequent PACs than those without (50% [8/16] vs. 22% [11/50], p < 0.05). Frequent PACs were associated with AF detection and time to the first AF after adjustment for CHADS2 score after index stroke, high plasma B-type natriuretic peptide (BNP; >100 pg/mL) or serum N-terminal pro-BNP levels (>300 pg/mL), and large left atrial diameter (≥45 mm). Conclusion: High frequency of PACs in cryptogenic stroke may be a strong predictor of AF detected by ICM.