Fibromyalgia (FM) is a chronic illness that does not have clear physical consequences, yet research shows that FM patients often have a low body image. An online cross-sectional study was conducted in Flanders, Belgium, among FM women who are connected to the Flemish League for fibromyalgia patients. An adjusted Body Image Scale (BIS) was used to assess body image, the General Health Questionnaire-12 (GHQ-12) was used for mental health, and the Visual Analogue Scale Fibromyalgia Impact Questionnaire (VASFIQ) was used for FM symptoms. Medication use was assessed by using a 4-point Likert scale. Time since diagnosis and age was assessed. A total of 103 women with FM responded. Linear regression showed that BIS was best predicted in a model by using VASFIQ, GHQ-12, time since diagnosis, and sleep medication, wherein only the GHQ-12 was significant as a variable (B = 0.292; p = 0.009). This model explained 19.3% of the variance. The role of sleep medication use disappeared when controlling for mental health. Mental health was more clearly associated with body image than medication use, or even fibromyalgia symptoms. Thus, having negative mental health is associated with a negative body image. In order to improve the body image of FM patients, symptom control alone is not enough; improving mental health is equally important.