Up to 27% of children with autism spectrum disorder (ASD) experience clinically significant externalizing behavior. Child emotional dysregulation is thought to be one of the main reasons for the manifestation of externalizing behaviors during toddlerhood and has also been associated with insecure and disorganized parent-child attachment relationships. Parent-Child Interaction Therapy for Toddlers (PCIT-T) is an attachment and behavioral based parent training program targeted at children aged 12 to 24 months with externalizing behavior. This study assessed the effectiveness of PCIT-T in reducing child externalizing behavior as well as improving child emotional regulation and the parent-child attachment relationship in a child with moderate-to-severe ASD traits, low intellectual and adaptive functioning, externalizing behaviors within the clinical range, and a disorganized/insecure mother-child attachment relationship. The intervention occurred over a 12-week period and focused on improving positive parenting skills, parent emotional regulation, and child emotional regulation. Treatment progress was tracked using the Child Behavior Checklist Externalizing Behavior subscale, the Deverux Early Childhood Assessment for Toddlers Attachment/Relationship and Self-Regulation subscales, the Dyadic Parent-Child Interaction Coding System, version 4, and the Strange Situation Procedure. Study results provide preliminary support for the use of PCIT-T in reducing externalizing behavior, improving the parent-child attachment relationship and child emotional regulation. These results indicate that PCIT-T can be beneficial for children with ASD traits, however, further research with a larger sample size is needed to strength these findings.