Peroxisome proliferator-activated receptor α (PPARα) controls hepatic lipid homeostasis and is the target of lipid-lowering fibrate drugs. PPARα activation represses expression of let-7 microRNA (miRNA), but the function of let-7 in PPARα signaling and lipid metabolism is unknown. In the current study, a hepatocyte-specific let-7b/c2 knockout (let7b/c2^ΔHep) mouse line is generated, and these mice are found to exhibit pronounced resistance to diet-induced obesity and fatty liver. Let-7 inhibition by hepatocyte-specific let-7 sponge expression shows similar phenotypes as let7b/c2^ΔHep mice. RNA sequencing (RNA-seq) analysis reveals that hepatic PPARα signaling is repressed in let7b/c2^ΔHep mice. Protein expression of the obligate PPARα heterodimer partner retinoid X receptor α (RXRα) is reduced in the livers of let7b/c2^ΔHep mice. Ring finger protein 8 (Rnf8), which is a direct target of let-7, is elevated in let7b/c2^ΔHep mouse liver and identified as a E3 ubiquitin ligase for RXRα. This study highlights a let-7-RNF8-RXRα regulatory axis that modulates hepatic lipid catabolism.