Although nonregular workers experience higher job stress, poorer mental health, and different job stress dimensions relative to regular workers, little is known about which job stress dimensions are associated with poor mental health among nonregular workers. This study investigated the association between job stress dimensions and mental health among Korean nonregular workers. Data were collected from 333 nonregular workers in Seoul and Gyeonggi Province, and logistic regression analysis was conducted. Results of the study indicated that high job insecurity and lack of rewards had stronger associations with poor mental health than other dimensions of job stress when controlling for sociodemographic and psychosocial variables. It is important for the government and organizations to improve job security and reward systems to reduce job stress among nonregular workers and ultimately alleviate their mental health issues.