The current study investigated overregularization of Spanish irregular past participles (e.g., dicho ‘said’, regularized as decido) among 20 child heritage speakers of Spanish in New Mexico, ages 5;1–11;9. Overregularization occurs when a child produces an irregular form analogously to its regular counterpart (e.g., eated instead of ate). Typically, children first produce the irregular form and then, after they have learned a morphological pattern, they overapply it to the irregular form. Ultimately, children retreat from overregularization and once again produce the target irregular form. While there has been a wealth of studies on monolingual children’s overregularizations, very few have investigated this phenomenon in child heritage speakers, who may develop their grammars diversely due to their exposure to the heritage language. This study analyzed the impact of age, Spanish language experience, Spanish morphosyntax proficiency, and lexical frequency on overregularization among the 13/20 children who produced past participles (n = 233) in response to an elicited production task. Participles were overregularized at high rates (74%), resulting in forms like ponido (‘put’, cf: puesto). Results from a regression analysis indicate that overregularization was more likely among the younger children, the children with lower morphosyntax scores, and with lower-frequency participles. Further, an interaction between morphosyntax score and lexical frequency indicated that children with higher scores overregularized with lower frequency participles, but not higher frequency ones, whereas children with low scores overregularized with both low- and high-frequency forms. In summary, child heritage speakers overregularize Spanish past participles at high rates, and the retreat from overregularization is tied to overall grammatical development and lexical frequency, suggesting that the acquisition of irregular participles is dependent on experiencing multiple instances of the irregular verb form.