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This paper presents a reliability-based design optimization (RBDO) method for a monopile transition piece in an offshore wind turbine system. Two design approaches are investigated for the cost-effective and reliable design of the monopile transition piece: deterministic optimization (DO) and RBDO. First, dynamic response analysis of a reference offshore wind turbine is conducted to estimate design loads considering site conditions off the southwest coast of Korea. Second, DO for minimizing the mass of a conical monopile connection is carried out, including an assessment of the reliability of the DO design. Next, RBDO is performed to achieve a design with the desired reliability while concurrently minimizing the mass of the monopile transition piece. The present study shows that the structural design of the monopile connection is mostly dictated by the fatigue limit state and that DO does not guarantee structural reliability even though the design satisfies all limit state function conditions. The proposed RBDO process is shown to speed up the design cycle and enhance the reliability of the grouted connection for offshore wind turbine support structures. [ABSTRACT FROM AUTHOR]