This is a Phase 1/Phase 2 open-label single arm, multicenter, and multinational study with SAR446268 for treatment of male and female participants 10 to 50 years old with non-congenital myotonic dystrophy (DM) type 1 (DM1). The purpose of this study is to evaluate the safety and efficacy of SAR446268 in knocking down dystrophia myotonica protein kinase (DMPK) messenger ribonucleic acid (mRNA) levels and improving neuromuscular function in DM1 participants receiving a single intravenous (IV) administration of SAR446268. The study consists of a dose escalation part (Part A) during which single ascending doses of SAR446268 will be evaluated in 3 distinct cohorts and an optional 4th dose cohort. Once a safe and effective dose is identified, additional participants will be treated in Part B, the dose expansion phase of the study. The study duration will be 110 weeks (approximately 2 years) for each participant in Parts A and B respectively and includes a 6-week screening phase and a 104-week follow-up period post-SAR446268 administration.