Toddlers' interactions with teachers, peers and the classroom environment are critical for their academic and social development. The newly developed measure – Individualized Classroom Assessment Scoring System for Toddlers (inCLASS Toddler) – evaluated the quality of toddlers' interactions in two European countries. This first validation study examined the structural, construct and criterion validity of the inCLASS Toddler. Using observational data of Danish and Dutch toddlers (N = 211) across 58 classrooms, the hypothesized four-domain structure showed an adequate and marginal fit to the Danish and Dutch data, respectively. Construct validity indicated within-country differences for children's age, but not for gender. Criterion validity was evidenced by small to large relations with children's social-emotional, language, and math skills. These results support the applicability of the inCLASS Toddler for observing toddlers' situated skills in interacting with their environment, which can be useful for practitioners and researchers to evaluate toddlers' interactions in an ecologically valid way.