Recent studies highlight the important contribution of phonological working memory (PM) in the early stages of both native and foreign language development. However, research on the effects of PM training on language development is very limited. This study aimed at assessing the effectiveness of a PM training educational intervention as a means of fostering vocabulary development in beginner-level young learners of English as a second/foreign language (L2). A double-blind pretest-posttest quasi-experimental design was adopted, with an experimental group (n = 50) and a matched active control group (n = 47). All participants were initially assessed with an English-sounding nonword repetition test and an English language vocabulary test (receptive and productive). In addition, the experimental group students participated in the PM training (33 sessions of 15-minutes length within 12 weeks), while the control students participated in non-phonological-memory related English language activities. After the conclusion of the intervention, PM and L2 vocabulary were reassessed in both groups. Results confirm previous findings on the significant relationship between PM and L2 vocabulary size and provide evidence for PM trainability, as well as on resultant L2 productive vocabulary gains. No effect of PM training was detected on receptive vocabulary development. Results are discussed with regard to their theoretical implications, and to possible applications of PM training as a method for supporting vocabulary development in the L2 classroom.