This is a Phase 3 study to evaluate the efficacy, safety, and tolerability of crinecerfont versus placebo administered for 24 weeks in approximately 165 adult participants with classic CAH due to 21-hydroxylase deficiency. The study consists of a 24-week randomized, double-blind, placebo-controlled period, followed by 1 year of active treatment with crinecerfont. Subsequently, participants may elect to participate in the open-label extension (OLE) period. The duration of participation in the study is approximately 20 months for the core study and will be a variable amount of time per participant for the OLE (estimated to be approximately 3 years).