The present prospective follow-up study aimed to evaluate the effects of GRK5 polymorphisms on ritodrine efficacy and adverse drug events (ADEs) in pregnant women undergoing preterm labor. A total of 162 women undergoing preterm labor were included in the study. Seven single nucleotide polymorphisms (SNPs) in the GRK5 gene (rs915120, rs2230345, rs2230349, rs7923896, rs1020672, rs4752308, and rs4752292) were assessed. Homozygous variant carriers of rs4752292 and rs1020672 had 0.6 times the hazard of delivery compared to wild-type allele carriers (95% confidence interval [CI], 0.41~0.99 and 0.38~0.99, respectively). In addition, homozygous variant carriers of rs4752292 and rs1020672 had 2.4-fold more (95% CI, 1.10~4.98) and 2.3-fold more (95% CI, 1.04~5.06) ADEs compared to those with the wild-type homozygotes, respectively. Among demographic variables, gestational age and modified Bishop score were significant factors associated with time to delivery, while body weight and maximum ritodrine infusion rate were significant factors associated with ADEs. In silico analysis showed that both rs4752292 and rs1020672 had the potential to affect mRNA splicing by alteration of splicing motifs. The present study shows that ritodrine efficacy and ADEs are associated with GRK5 gene polymorphisms in pregnant women undergoing preterm labor.