In modular structures, prefabricated modular units are joined at the construction site. Modular structures must ensure splicing performance by connecting modular units sufficiently. The bolted connection using steel plates may suffer from alignment issues and corrosion problems. In a precast concrete (PC) modular system, there is difficulty grouting the sleeves when splicing reinforcing bars. This study proposed a PC modular beam using a bolted connecting plate to deal with issues in typical steel modules and PC modules. The structural performance was evaluated by flexural and shear tests on two monolithic beams and two proposed PC specimens. The test results showed that the structural performance of the PC modular specimen was 88% of that of the monolithic reinforced concrete (RC) beam specimen and 102% of the strength calculated by ACI 318-19. Therefore, the proposed PC modular system using bolted connecting plates can solve the problems observed in typical steel and PC modules and improve the structural performance.