Chikungunya virus (CHIKV), a mosquito-borne alphavirus, causes millions of infection globally. Posaconazole (PCZ) is an antifungal drug, which we and others have previously found to inhibit replication of a number of viruses, including dengue virus, a member of the Flaviviridae family. In this study, we analyzed the antiviral activity of PCZ against alphaviruses. We found that PCZ potently inhibits a number of alphaviruses, including Semliki forest virus (SFV), Sindbis virus (SINV) and CHIKV with half maximal effective concentration (EC50) of 2.3 μM, 4.0 μM and 0.8 μM, respectively. Time-of-addition assays indicated that PCZ treatment before and at the time of SFV infection showed potent inhibition, whereas addition of PCZ at later time points post infection showed minor to no inhibition, suggesting inhibition at an early stage of the replication cycle. In accordance, PCZ treatment of a temperature sensitive mutant of SFV that is capable of cell entry and translation, but not RNA replication, resulted in an almost 90% reduction in luciferase activity. To confirm these findings, PCZ resistant mutant virus were generated and we identified mutations in E1 (V148A) and E2 (H255R) viral glycoproteins, of which the E2 mutation confers partial resistance to PCZ when introduced into wild-type SFV. To see whether PCZ alters clathrin-mediated endocytosis, we analyzed the uptake of fluorescence-tagged transferrin and found that PCZ reduced transferrin uptake by 50% compared to DMSO-treated cells. Together, these results establish PCZ as a novel inhibitor of alphaviruses and identify viral entry as its target.