Vocabulary experts recommend first language (L1) translation equivalents for establishing form-meaning mappings for new second language (L2) words, especially for lower proficiency learners. Empirical evidence to date speaks in favor of L1 translation equivalents over L2 meaning definitions, but most studies have investigated bi-rather than multilingual learners. In our study, we investigated instructed English vocabulary learning through an intervention study in six language-diverse secondary school English classrooms in Sweden (N = 74) involving three conditions for presentation of word meanings: (a) definitions in the L2 (English), (b) translation equivalents in the shared school and majority language (Swedish), and (c) translation equivalents in the shared school and majority language plus other prior languages among the learners (Swedish and other). Based on overall weighted mean effect sizes and mixed-effects A one-page Accessible Summary of this article in non-technical language is freely available in the Supporting Information online and at https://oasis-database.orgWe gratefully acknowledge funding from The Swedish Research Council, Project ID 2016-03469. Our gratitude extends to the participating teachers and students as well as to school leaders and staff who contributed to making our research possible. We are also very grateful to the Journal Editor Emma Marsden for exceptionally helpful feedback and recommendations and to five anonymous reviewers for their constructive comments and suggestions during the review process. Any remaining flaws are our own responsibility.