This study investigated the longitudinal relation between children's theory of mind and their metacognitive knowledge with a special focus on the impact of language on this relation. Measures of theory-of-mind understanding and metamemory knowledge were assessed in 133 3-year-old children at 4 measurement points separated by half-year intervals. Results revealed a somewhat more pronounced effect of children's early theory of mind on the change in metamemory knowledge than vice versa. This effect was almost independent of the time period under investigation as well as of earlier language and nonverbal competencies and socioeconomic status. Early language competencies had a direct impact on theory-of-mind and metamemory development and moreover indirectly affected metamemory development through children's theory of mind. The results of the study suggest that children's abilities to metarepresent together with their early language competencies are important in the acquisition of metamemory knowledge.