Updated project metadata. Many mammalian microRNAs are embedded within introns of protein-coding genes, yet their functional relationship with the host gene is poorly understood. Here we identify Mir483, as having opposing developmental and physiological roles to those of its host gene, the paternally-expressed Igf2. Mechanistically, Mir483 expression is dependent on Igf2 transcription and the epigenetic regulation of the Igf2/H19 imprinting control region. Mir483 deletion in vivo resulted in normal growth, but induced distinct molecular signatures. Transgenic Mir483 overexpression in utero caused fetal, but not placental, growth restriction and cardiovascular defects leading to fetal demise. Overexpression of Mir483 postnatally resulted in growth stunting through IGF1 repression, increased hepatic lipid production, and excessive adiposity. IGF1 infusion rescued the post-natal growth restriction. Our data highlight a novel functional antagonism between a growth-suppressor microRNA and its growth-promoting host gene. We suggest that Mir483 was evolutionary co-opted to provide exquisite control of IGF signalling activity.