Multilingual children living in areas with low socioeconomic status (SES) are at an increased risk of having speech, language and communication needs (SLCN). Many multilingual children with SLCN in Sweden have insufficient basic vocabulary knowledge in the majority language, preventing them from achieving the school's learning goals. However, there is a lack of studies of interventions that could improve vocabulary development in this group, and that can be delivered in an everyday educational setting. This pilot study evaluates the effect of a school-based vocabulary group intervention, which combines elements from shared book reading and the Lexicon Pirate method, on vocabulary skills. A non-randomised pretest-posttest control group design was used. Eighteen six- to seven-year-old multilingual children attending preschool class in low SES areas either received intervention (n = 9) or were included in a control group (n = 9). The intervention group participated in 12 intervention sessions over six weeks. Word knowledge, semantic fluency, and comprehension of semantic relations were assessed pre- and post-intervention. Results revealed that children in the intervention group demonstrated significant gains in knowledge of both trained and untrained words, compared to the control group. No effects were found on semantic fluency or comprehension of semantic relations.