The main purpose of this study is to see how well different doses of soquelitinib work in participants with atopic dermatitis (AD) as compared to a placebo (pill with no medicine in it). To check this, the study doctors will examine participants' skin regularly to track how their AD changes during the study.

The study doctors will see how safe soquelitinib is by tracking side effects among participants. They will also check "tolerability," which means how well the participants can handle soquelitinib and whether any potential side effects are manageable for them.

To understand how well soquelitinib works, it will be compared with a placebo. Placebo is a substance that looks like soquelitinib but contains no active medicine in it.

Participants will:

• Take study treatment (soquelitinib or placebo) every day for 12 weeks
• Visit the clinic for check-ups and tests every week for the first 2 weeks then every 2 weeks during the treatment period, and then return for follow-up visits 30, 60, and 90 days after the last dose of study treatment

Inclusion Criteria:

• Adult male or female, ≥18 years of age, as applicable according to local legislation, at Screening.
• Diagnosis of AD with onset of symptoms at least 1 year prior to the Screening visit.
• Moderate to severe disease at screening and pre-randomization on Day 1 defined by:
• EASI ≥16
• Body surface area ≥10%
• vIGA ≥3
• PP-NRS average score of ≥4 over a period of 7 days prior to randomization at Day 1.
• Documented history of prior topical and/or systemic therapy for AD within 6 months of randomization.
• A female participant is eligible if she is not pregnant or breastfeeding, and is either of non-childbearing potential, OR of childbearing potential and agrees to use highly effective birth control

Exclusion Criteria:

• Other concomitant skin conditions that would interfere with evaluations of the effect of study medication on AD.
• Other active skin diseases or skin infections (bacterial, fungal, or viral) requiring systemic treatment within 30 days of the Baseline visit or that could interfere with the appropriate assessment of AD lesions.
• Known immunodeficiency syndrome, history of an invasive infection or active non skin-related infection requiring systemic anti-infective treatment within 30 days of Baseline.
• History of immunosuppression not related to medication, history of clinically significant medical conditions, or any other reason which in the opinion of the investigator would interfere with the participant's participation in this study.
• Have an unstable or uncontrolled illness, including but not limited to cerebrocardiovascular, respiratory, gastrointestinal, endocrine, hematologic, or neurologic disorders that would potentially affect participant safety within the study or confound efficacy and safety assessments.