The purpose of the study is to assess the efficacy, safety, and tolerability of zeleciment basivarsen (DYNE-101) for the treatment of myotonic dystrophy 1 (DM1).

The study consists of three periods: a Screening period (up to 8 weeks), Placebo-Controlled Period (48 weeks) and a Long-Term Extension Period (24 weeks).

An Independent Data Monitoring Committee (IDMC) comprised of members independent and external to the Sponsor will review safety and tolerability data of this study at regular intervals.

Inclusion Criteria:

• Diagnosis of DM1 confirmed by molecular genetics with trinucleotide repeat size greater than (>) 100. Historical results from clinical testing are acceptable.
• Able to walk 10 meters and complete 5 times sit to stand independently (inserts or supports that don't go above the ankle are allowed).
• Body mass index (BMI) less than (<) 35 kilograms per meter square (kg/m^2).

Exclusion Criteria:

• A known diagnosis of congenital DM1.
• History of major surgical procedure (based on Investigator judgment) within 12 weeks prior to the start of screening, with the exception of implanted pacemaker or defibrillator.
• Use of glucagon-like peptide 1 (GLP-1) agonist/incretin medications including semaglutide, dulaglutide, liraglutide, exenatide, or tirzepatide within a period of 5 half-lives of the medication prior to performing screening assessments.

Note: Other inclusion and exclusion criteria may apply.