A seat backrest is provided, comprising a framework (1) including an upper portion (10), a lower portion (20), a central support portion (40) and two side portions (30) formed integrally by injection molding a thermoplastic material; and an elongated reinforcement (60) embedded in the side portion (30) and the central support portion (40) by overmolding and extending along the side portion (30), the reinforcement (60) being configured to bend along a boundary (35) between the side portion (30) and the central support portion (40). A seat including the backrest is also provided.