Part A and B participants are eligible to be included in the study only if all of the following criteria apply: 1. Male or female at birth, between 1 to <18 years of chronological age at the time of signing the Informed Consent Form (ICF). 2. Have a medically confirmed diagnosis of classic CAH due to 21-hydroxylase deficiency (21-OHD) based on standard medically accepted criteria such as elevated 17-OHP level, confirmed CYP21A2 genetic testing, positive newborn screening with confirmatory second tier testing, or cosyntropin stimulation. 3. Participants must have an elevated morning serum A4 level >ULN during Screening obtained prior to morning glucocorticoid (GC) administration. 4. Participants must be on a stable supraphysiologic GC replacement therapy for at least one month prior to Screening. 5. Compliance, as judged per Investigator discretion, with GC replacement and mineralocorticoid replacement (if applicable) regimen documented during the Screening Period. 6. Normal thyroid stimulating hormone (TSH) and thyroxine (T4) within 3 months of Screening per age-appropriate range. Part C inclusion criteria require participants to complete treatment in either Part A or Part B and in the Investigator's opinion it would benefit the participant to continue in Part C, regardless of age.

Part A and Part B: Individuals in Part A and Part B who meet any of the following criteria will be excluded from participation in this study: 1. Diagnosis of any form of CAH other than classic 21-OHD. 2. Participants treated with other GCs within 30 days of Screening. 3. Stress dose of GC therapy within 2 weeks of start of Screening, defined as any dose above the normal maintenance dose, including but not limited to intravenous (IV) or intramuscular (IM) hydrocortisone. 4. Use of growth hormones within 1 week of start of Screening for short acting, or within 6 weeks of start of Screening for long acting. 5. Use of a corticotropin-releasing factor receptor antagonist within 14 days of Screening. 6. History of cancer excluding cured/treated dermal squamous or basal cell carcinoma or cervical carcinoma in situ. 7. Abnormal sleep/wake cycles (as determined by the Investigator). 8. Female participants who are pregnant or lactating. 9. Participants who have been dosed with an investigational drug (other than atumelnant) in any prior clinical study within 60 days or 5 half-lives (whichever is longer) prior to the first dose. Part C: 10. Individuals in Part C who do not meet the Part C Inclusion Criteria.