Background: Parent-child interaction therapy (PCIT) is an evidence-based approach typically used for children aged 2-7 with externalizing disorders. Research suggests that PCIT is effective in treating a broad range of populations and problems. Several adaptations of PCIT have been developed for its use with children younger than the age of 2, though it is unclear how efficacious these adaptations are. Given the differences in terminology used in the developmental adaptations of PCIT, the current meta-analysis defines 12-24-month-olds as younger preschoolers and 25-59-month-olds as older preschoolers. Objectives: The current meta-analysis aims to synthesize the literature on PCIT with children younger than the age of 5 and compare the effect of PCIT for younger preschoolers and older preschoolers. Methods: Searches of PsycINFO and PubMed yielded 30 studies meeting inclusion criteria which were aggregated via meta-analysis. Results: Statistical analyses suggest that PCIT overall has a large aggregate effect size when examining child externalizing symptoms, child internalizing symptoms, caregiver distress, and caregiver behaviors. PCIT interventions for older preschoolers were found to be comparably effective as PCIT adaptations for younger preschoolers in improving child externalizing symptoms, caregiver distress, and caregiver behaviors. Conclusions: Overall, this meta-analysis provides preliminary findings that PCIT adaptations for younger preschoolers are effective in improving the well-being of children and caregivers. These adaptations may have benefits beyond the outcomes examined in this meta-analysis and future studies should continue to focus on the effects of these adaptations.