The purpose of this study is to evaluate the safety, tolerability, pharmacokinetics (PK) and pharmacodynamics (PD) of ARO-DUX4 in participants with facioscapulohumeral muscular dystrophy Type 1 (FSHD1). In Part 1 of the study, participants will receive one dose of ARO-DUX4 or placebo. In Part 2 of the study, participants will receive 4 doses of ARO-DUX4 or placebo. Participants who complete Part 1 will have the option to re-screen and re-randomize into Part 2. All participants will undergo pre- and post-dose MRI-guided muscle biopsies (a total of 2 biopsies). Participants who complete Part 1 and enroll in Part 2 will be required to undergo an additional screening biopsy. Participants completing Part 1 or Part 2 may have the option to continue to receive drug in an open-label extension study or may be eligible to participate in later-stage clinical studies.

Inclusion Criteria:

• Genetically confirmed FSHD1 based on Screening evaluation or source verifiable medical record
• Clinical severity score between 3 and 8 (scale, 0 to 10)
• Must have eligible lower extremity muscle for biopsy as determined from MRI by a central reader
• A 12-lead electrocardiogram (ECG) at Screening with no abnormalities that may compromise participant's safety in the study
• Participants of childbearing potential and their partners must use highly effective contraception during the study and for at least 12 weeks following the end of study or last dose of study medication, whichever is later. Males must not donate sperm during the study from Day 1 until at least 12 weeks following the end of study or last dose of study medication, whichever is later.

Exclusion Criteria:

• Human Immunodeficiency Virus (HIV) infection as shown by presence of anti-HIV antibody (seropositive) at Screening
• Seropositive for hepatitis B (HBV) or hepatitis C (HCV) at Screening
• Uncontrolled hypertension
• Severe cardiovascular disease
• History of thrombolic events
• Platelet count less that the lower limit of normal at Screening
• History or presence of: a hypercoagulable state, nephrotic range proteinuria, antiphospholipid antibody syndrome, myeloproliferative disease, inability to ambulate, use of hormone-based contraceptives.
• Any contraindication to muscle biopsy or MRI

Note: additional inclusion/exclusion criteria may apply per protocol