MicroRNAs (miRNAs) play critical roles in diverse cellular events through their effects on translation. Emerging data suggest that modulation of miRNA biogenesis at post-transcriptional steps by RNA-binding proteins is a key point of regulatory control over the expression of some miRNAs and the cellular processes they influence. However, the extent and conditions under which the miRNA pathway is amenable to regulation at posttranscriptional steps are poorly understood. Here we show that RBM3, a cold-inducible, developmentally regulated RNA-binding protein and putative protooncogene, is an essential regulator of miRNA biogenesis. Utilizing miRNA array, Northern blot, and PCR methods, we observed that over 60% of miRNAs detectable in a neuronal cell line were significantly downregulated by knockdown of RBM3. Conversely, for select miRNAs assayed by Northern blot, induction of RBM3 by overexpression or mild hypothermia increased their levels. Changes in miRNA expression were accompanied by changes in the levels of their ~70 nt precursors, whereas primary transcript levels were unaffected. Mechanistic studies revealed that knockdown of RBM3 does not reduce Dicer activity or impede transport of pre-miRNAs into the cytoplasm. Rather, we find that RBM3 binds directly to ~70 nt pre-miRNA intermediates and promotes / de-represses their ability as larger ribonucleoproteins (pre-miRNPs) to associate with active Dicer complexes. Our findings suggest that the processing of a majority of pre-miRNPs by Dicer is subject to an intrinsic inhibitory influence that is overcome by RBM3 expression. RBM3 may thus orchestrate changes in miRNA expression during hypothermia and other cellular stresses, and in the euthermic contexts of early development, differentiation, and oncogenesis where RBM3 expression is highly elevated. Additionally, our data suggest that temperature-dependent changes in miRNA expression mediated by RBM3 may contribute to the therapeutic effects of hypothermia, and are an important variable to consider in in vitro studies of translation-dependent cellular events.