Following the 2022 global mpox outbreak, Brazil has experienced sustained low-level transmission driven by MPXV clade IIb. Since 2024, clade Ib has emerged globally, with increasing evidence of community transmission. We report early evidence consistent with local transmission of MPXV clade Ib in Rio de Janeiro, Brazil. In a prospective cohort at INI-Fiocruz, 23 people were diagnosed with mpox between March and April 2026, including 14 clade Ib infections (13 analyzed). All participants were cisgender men (median age 33 years), predominantly reporting sex with men, without recent international travel. Phylogenetic analysis identified a well-supported cluster consistent with local transmission, nested within a European lineage, suggesting recent introduction into Rio de Janeiro in early 2026, with onward spread. From April 7, all new diagnoses were clade Ib. Clinical features overlapped with clade IIb, limiting syndromic differentiation. Complications were frequent, and five required hospitalizations, highlighting need for enhanced surveillance and prevention.