Background: The number of words children produce (expressive vocabulary) and understand (receptive vocabulary) changes rapidly during early development, partially due to genetic factors. Here, we performed a meta–genome-wide association study of vocabulary acquisition and investigated polygenic overlap with literacy, cognition, developmental phenotypes, and neurodevelopmental conditions, including attention-deficit/hyperactivity disorder (ADHD). Methods: We studied 37,913 parent-reported vocabulary size measures (English, Dutch, Danish) for 17,298 children of European descent. Meta-analyses were performed for early-phase expressive (infancy, 15–18 months), late-phase expressive (toddlerhood, 24–38 months), and late-phase receptive (toddlerhood, 24–38 months) vocabulary. Subsequently, we estimated single nucleotide polymorphism–based heritability (SNP-h²) and genetic correlations (r_g) and modeled underlying factor structures with multivariate models. Results: Early-life vocabulary size was modestly heritable (SNP-h² = 0.08–0.24). Genetic overlap between infant expressive and toddler receptive vocabulary was negligible (r_g = 0.07), although each measure was moderately related to toddler expressive vocabulary (r_g = 0.69 and r_g = 0.67, respectively), suggesting a multifactorial genetic architecture. Both infant and toddler expressive vocabulary were genetically linked to literacy (e.g., spelling: r_g = 0.58 and r_g = 0.79, respectively), underlining genetic similarity. However, a genetic association of early-life vocabulary with educational attainment and intelligence emerged only during toddlerhood (e.g., receptive vocabulary and intelligence: r_g = 0.36). Increased ADHD risk was genetically associated with larger infant expressive vocabulary (r_g = 0.23). Multivariate genetic models in the ALSPAC (Avon Longitudinal Study of Parents and Children) cohort confirmed this finding for ADHD symptoms (e.g., at age 13; r_g = 0.54) but showed that the association effect reversed for toddler receptive vocabulary (r_g = −0.74), highlighting developmental heterogeneity. Conclusions: The genetic architecture of early-life vocabulary changes during development, shaping polygenic association patterns with later-life ADHD, literacy, and cognition-related traits.