This Phase 3, multicenter, randomized, double-blind, placebo-controlled study will evaluate the efficacy, safety, and tolerability of SAB-142, a fully human anti-thymocyte globulin (h-ATG), in participants aged 5 to 40 years with Stage 3 type 1 diabetes (T1D). The study will enroll participants with recent-onset T1D (\>100 days to \<1 year from diagnosis) and established-onset T1D (≥1 year to ≤2 years from diagnosis) who retain residual beta-cell function as demonstrated by stimulated C-peptide levels \>0.2 nmol/L. Participants will be randomized in a 2:1 ratio to receive SAB-142 or placebo in addition to standard diabetes care. The primary objective is to determine whether SAB-142 preserves beta-cell function over 12 months as measured by stimulated C-peptide response during a mixed meal tolerance test (MMTT). External data from the SAB-142-201 SAFEGUARD study will be incorporated to include participants with new-onset T1D (\<100 days from diagnosis) in the primary efficacy analysis.