In this paper, we describe the screening of a 14640-compound library using a novel whole mycobacteria phenotypic assay to discover inhibitors of EthR, a transcriptional repressor implicated in the innate resistance of Mycobacterium tuberculosis to the second-line antituberculosis drug ethionamide. From this screening a new chemical family of EthR inhibitors bearing an N-phenylphenoxyacetamide motif was identified. The X-ray structure of the most potent compound crystallized with EthR inspired the synthesis of a 960-member focused library. These compounds were tested in vitro using a rapid thermal shift assay on EthR to accelerate the optimization. The best compounds were synthesized on a larger scale and confirmed as potent ethionamide boosters on M. tuberculosis -infected macrophages. Finally, the cocrystallization of the best optimized analogue with EthR revealed an unexpected reorientation of the ligand in the binding pocket.