To explore aromatase inhibition and to broaden the structural diversity of dual aromatase-sulfatase inhibitors (DASIs), we introduced the steroid sulfatase (STS) inhibitory pharmacophore to letrozole. Letrozole derivatives were prepared bearing bis-sulfamates or mono-sulfamates with or without adjacent substituents. The most potent of the achiral and racemic aromatase inhibitor was 40 (IC50 = 3.0 nM). Its phenolic precursor 39 was separated by chiral HPLC and the absolute configuration of each enantiomer was determined using vibrational and electronic circular dichroism in tandem with calculations of the predicted spectra. Of the two enantiomers, (R)-phenol (39a) was the most potent aromatase inhibitor (IC50 = 0.6 nM, comparable to letrozole), whereas the (S)-sulfamate, (40b) inhibited STS most potently (IC50 = 553 nM). These results suggest that a new structural class of DASI for potential treatment of hormone-dependent breast cancer has been identified, and this is the first report of STS inhibition by an enantiopure nonsteroidal compound.