Elevated dihydrotestosterone (DHT), generated by the reduction of testosterone through steroid 5α-reductase, is associated with benign prostatic hyperplasia (BPH), acne, hirsutism, and androgenic alopecia. While several steroidal (e.g., finasteride, SKF 106687) and nonsteroidal (e.g., ONO-38057) 5α-reductase inhibitors exist, two isozymes (types I and II) encoded by distinct genes have been identified. Herein, we report a novel class of benzoquinolinones as potent and selective inhibitors of human type I 5α-reductase. Synthesized analogously to Cannon's method—starting from 2-tetralones via pyrrolidine enamine formation, condensation with acrylamide, methylation, and ionic reduction to yield cis/trans isomers—these compounds were evaluated in Hs68 human foreskin fibroblast cells (type I assay). The parent hexahydrobenzoquinolinone 6a had an IC50 of 6.5 μM; potency was enhanced by fluorine substitution (e.g., 8-fluoro 6b, IC50 0.6 μM), N-methylation, and octahydro (vs. hexahydro) structures, with trans isomers (e.g., 9a) more active than cis. Compound 9a (LY191704), an N-methylated trans octahydro derivative, was extremely potent (IC50 0.008 μM). Selectivity studies showed 9a inhibited type I (Hs68) with an IC50 of 8 nM but weakly inhibited type II (human prostate homogenate, IC50 >10,000 nM), whereas finasteride (1) favored type II (IC50 10 nM vs. 62 nM for type I) and SKF 106687 (2) was type II-selective (IC50 8 nM vs. 7500 nM for type I). In conclusion, benzoquinolinones are potent, selective human type I 5α-reductase inhibitors with low nanomolar IC50s, representing a novel opportunity to clinically target DHT overproduction via the type I enzyme. Importantly, this class achieves potent inhibition via a compact, rigid tricyclic nucleus without requiring a full steroid ring system.