Objective: YKL-40 is a highly conserved and chitin-bound human glycoprotein in mammals that is associated with airway inflammation and has no enzyme activity. We aimed to conduct a meta-analysis to assess the use of YKL-40 levels as a diagnosis of asthma. Methods: A meta-analysis was conducted based on the data from medical literature data base searches with time restrictions of January 2007 to January 2021. We searched and extracted relevant information from a total of 15 studies that reported YKL-40 levels in patients with asthma and in healthy controls, and obtained a sample of 1647 patients with asthma and 1259 healthy controls. Standardized mean differences (SMD) with 95% confidence intervals (CI) were calculated for this study by using statistical software packages. Results: Serum YKL-40 levels were higher in the patients with asthma than in the healthy controls (SMD 1.36 ng/ml [95% CI, 0.82-1.89 ng/ml]). YKL-40 levels are elevated in pediatric patients with asthma (SMD 2.26 ng/ml [95% CI, 1.33-3.18 ng/ml]) and in adult patients with asthma (SMD 0.96 ng/ml [95% CI, 0.26-1.66 ng/ml]). In addition, a subgroup analysis of asthma disease status showed that YKL-40 levels were significantly increased in the patients with stable asthma (SMD 1.69 ng/ml [95% CI, 0.81-2.56 ng/ml]) and in those with acute exacerbation asthma (SMD 3.31 ng/ml [95% CI, 2.04-4.58 ng/ml]), and serum YKL-40 levels were significantly higher in patients with acute exacerbation asthma than in patients with stable asthma (SMD 1.49 ng/ml [95% CI, 0.50-2.48 ng/ml]). Conclusion: Results of this meta-analysis suggested that increased serum levels of YKL-40 in patients with asthma could be used as an emerging indicator for distinguishing individuals with asthma from healthy individuals.