ABD0171 is a potent, irreversible, and selective inhibitor of aldehyde dehydrogenase 1A3 (ALDH1A3), with an IC50 of 0.34 µM using the physiological substrate all-trans-retinaldehyde (atRAL). It also inhibits ALDH1A2 and ALDH1A1 with IC50 of 0.79 µM and 6.6 µM, respectively. ABD0171 disrupts key oncogenic pathways in triple-negative breast cancer (TNBC) cells in vitro and exhibits robust antitumor and antimetastatic activity in vivo.