Polysemy, or the property of words having multiple meanings, is a prevalent feature of vocabulary. In this study we validated a new measure of polysemy knowledge for children with English as an additional language (EAL) and a first language (EL1) and examined the relationship between polysemy knowledge and age, language status, and reading comprehension. Participants were 112 British children aged 5 to 6 (n = 61) or 8 to 9 years (n = 51), 37% of whom had EAL (n = 41). Participants completed the new measure of knowledge of polysemes, along with other measures of language, literacy and cognitive ability. The new measure was reliable and valid with EAL and EL1 children. Age and language status predicted children's polyseme knowledge. Polyseme knowledge uniquely contributed to reading comprehension after controlling for age, language status, non-verbal intelligence, time reading in English, and breadth of vocabulary. This research underscores the importance of polysemy for children's linguistic development.