Hepatocellular carcinoma (HCC) is a cancer with multiple etiologies, and widespread prevalence. Largely refractory to current treatments, HCC is the fourth leading cause of cancer related deaths, world-wide. MicroRNAs (miRNAs) are important regulators in HCCs. We aimed to identify tumor suppressor miRNAs during tumor regression in a conditional c-MYC driven mouse model (LT2/MYC) of HCC, and to evaluate their therapeutic potential for HCC treatment METHODS: We performed miRNA expression profiling of developed and regressing LT2/MYC tumors, and in-depth in vitro gain- and loss-of-function analyses. Effect of adeno-associated virus vector (AAV)-mediated miR-342-3p treatment was evaluated in three HCC mouse models RESULTS: We identified miR-342-3p as a tumor suppressor miRNA in HCC, with increased expression in regressing tumors. Forced miR-342-3p expression in hepatoma cells showed significantly decreased cell proliferation, migration and colony formation. In vivo administration of AAV-miR-342-3p led to significant attenuation of tumor development and increased overall survival. We identified Mct1 as a bona fide target of miR-342-3p in HCC. We show the tumor suppressor role of miR-342-3p is executed partly by modulating the lactate transport function of MCT1. Importantly, we find miR-342-3p downregulated in tumors from HCC patients compared to matched non-tumor tissues, inversely correlating with MCT1 expression. We observed similar findings in TCGA-LIHC data.In our study, we identified and validated miR-342-3p as a tumor suppressor miRNA, in HCC. We demonstrated its therapeutic efficacy in significantly attenuating tumor development, and prolonging survival, in different HCC mouse models. Identification of miR-342-3p as an effective tumor suppressor opens a therapeutic avenue for miRNA-mediated attenuation of HCC development.