Background: Auxiliary tasks such as administrative work often include tasks that are unnecessary in the view of workers but still have to be done. These tasks can threaten a worker’s self-esteem. The purpose of this study was to examine the effects of unnecessary and unreasonable tasks on musculoskeletal pain. Methods: Fifty-five office workers (29 male; mean age = 41.96, SD = 14.2 years) reported their unnecessary and unreasonable tasks at the beginning of the study and kept a diary of their daily musculoskeletal pain over 5 weeks, using a visual analogue scale. Other work-related risk factors (prolonged sitting), job resources (participation in decision-making), and individual risk factors (sex, smoking, exercise, body mass index, maladaptive back beliefs) were controlled for in multilevel regression analysis. Findings: Multilevel regression analysis with 742 reports showed unnecessary tasks ( B = 4.27, p = .006)—but not unreasonable tasks ( B = 3.05, p = .074)—to predict the daily intensity of musculoskeletal pain, beyond other significant risk factors, such as prolonged sitting ( B = 2.06, p = .039), body mass index ( B = 1.52, p < .001), and maladaptive back beliefs ( B = 3.78, p = .003). Participation in decision-making was not a significant protective factor ( B = −1.67, p = .176). Conclusions/Application to Practice: The higher frequency of unnecessary tasks—compared with unreasonable tasks—could place workers at risk for musculoskeletal pain. Work redesign that reduces unnecessary and unreasonable tasks can make a valuable contribution to worker health and safety among office workers.