Control of translation is a fundamental source of regulation in gene expression. The induction of protein synthesis by brain-derived neurotrophic factor (BDNF) critically contributes to enduring modifications of synaptic function, but how BDNF selectively affects only a minority of expressed mRNAs is poorly understood. We report that BDNF rapidly elevates Dicer, increasing mature miRNA levels and inducing RNA processing bodies in neurons. BDNF also rapidly induces Lin28, causing selective loss of Lin28-regulated miRNAs and a corresponding upregulation in translation of their target mRNAs. Binding sites for Lin28-regulated miRNAs are necessary and sufficient to confer BDNF responsiveness to a transcript. Lin28 deficiency, or expression of a Lin28-resistant Let-7 precursor miRNA, inhibits BDNF translation specificity and BDNF-dependent dendrite arborization. Our data establish that specificity in BDNF-regulated translation depends upon a two-part posttranscriptional control of miRNA biogenesis that generally enhances mRNA repression in association with GW182 while selectively derepressing and increasing translation of specific mRNAs. Display omitted ► Brain-derived neurotrophic factor (BDNF) increases levels of Dicer and mature miRNAs ► BDNF also induces Lin28 and selective miRNA loss in terminally differentiated neurons ► BDNF-induced dendrite growth requires Lin28-mediated degradation of pre-Let-7 miRNAs ► Regulation of miRNA biogenesis underlies BDNF's target selection in protein synthesis Brain-derived neurotrophic factor selectively increases the translation of a subset of expressed mRNAs by regulating levels of both Dicer and Lin28 and, in this way, dictating the profile of neuronal miRNAs.