To address the high peak-to-average power ratio (PAPR) problem in orthogonal frequency division multiplexing with index modulation (OFDM-IM) systems, this paper proposes a novel PAPR reduction approach called the joint partial transmit sequence and index modulation mapping (J-IMM-PTS) scheme. The J-IMM-PTS method operates in two stages: the first stage applies index modulation mapping (IMM) rules to achieve an initial reduction in PAPR, while the second stage employs the partial transmit sequence (PTS) technique to further process signals that still exhibit high PAPR. A threshold mechanism is introduced to decide whether the PTS stage should be executed, enabling a balance between PAPR reduction performance and computational complexity. Simulation results demonstrate that the proposed J-IMM-PTS scheme achieves superior PAPR reduction compared to the traditional IMM method while also reducing computational complexity.