Purpose: This study explores whether the quality of parent-child interaction is associated with language abilities cross-sectionally and longitudinally up to preschool-age among children with developmental language disorder (DLD). Method: Participants were 97 monolingual children with DLD and their parents from the Helsinki Longitudinal SLI study, HelSLI (baseline, age in years;months, M = 4;3, SD = 0;10), of which 71 pairs were followed longitudinally (age in years;months, M = 6;6, SD = 0;5). Video recordings from three play sessions were scored for child, parent, and dyadic behavior using Erickson's sensitivity scale protocol and mutually responsive orientation at baseline. Children's expressive and receptive language and language reasoning ability were assessed at baseline, and expressive and receptive language were assessed at follow-up. Results: At baseline, engaged child behavior, parent's supportive guidance, and fluent and attuned dyadic behavior were associated with better receptive language ability, and engaged child behavior and dyadic synchrony were positively associated with language reasoning ability in 3- to 6-year-olds. The child's positive engagement and fluent and attuned dyadic behavior at baseline were associated with better expressive and receptive language abilities at follow-up in 6- to 7-year-olds, respectively. Conclusions: Fluent and attuned dyadic behavior is associated with better receptive language ability in preschool-age children. Parent behavior alone was not associated with language ability. A connected and mutually attuned parent--child relationship could be a protective factor for language development for children with DLD.