Reactive oxygen species (ROS) play dual roles in plant stress response, but how plants modulate the dual roles of ROS in stress response is still obscure. OsJAB1 (JUN-activation-domain-binding protein 1) encodes the rice CSN5 (COP9 signalsome subunit 5). This study showed that, similar to the Arabidopsis homolog gene CSN5B, OsJAB1-overexpressing (driven by a CaMV 35S promoter) plants (OEs) impaired rice salt stress tolerance; in contrast, OsJAB1-inhibited-expression (using RNA-interfering technology) plants (RIs) enhanced rice salt stress tolerance. Differing from CSN5B that negatively regulated ascorbate (Asc) biosynthesis, Asc content increased in OEs and decreased in RIs. ROS analysis showed that RIs clearly increased, but OEs inhibited ROS accumulation at the early stage of salt treatment; in contrast, RIs clearly decreased, but OEs promoted ROS accumulation at the late stage of salt treatment. The qPCR revealed that OEs decreased but RIs enhanced the expressions of ROS-scavenging genes. This indicated that OsJAB1 negatively regulated rice salt stress tolerance by suppressing the expression of ROS-scavenging genes. This study provided new insights into the CSN5 homologous protein named OsJAB1 in rice, which developed different functions during long-term evolution. How OsJAB1 regulates the Asc biosynthesis that coordinates the balance between cell redox signaling and ROS scavenging needs to be investigated in the future.