Chronic pain is a major health problem. It causes high economic and social costs around the world and severely impairs the quality of life of those who suffer from it. Chronic pain and major depression frequently co-occur. Patients with both conditions have a worse prognosis and higher disability, and their treatment options are scarce. Behavioral activation (BA) may be an especially useful intervention for these patients. This intervention targets mechanisms of action that seem to be common to both disorders. In spite of this, the efficacy of this intervention has not been yet examined in people with both conditions. Therefore, the purpose of the present study is to examine the efficacy of BA compared to usual care among Chilean women with fibromyalgia and mayor depression (N = 90). Women will be randomized to an experimental arm (n = 45) who will receive usual care (UC) for fibromyalgia with comorbid depression plus BA; and a comparison arm, who will receive only UC for fibromyalgia with comorbid depression (n =45). Primary and secondary outcomes will be assessed before, during, and after the intervention, as well as at a three month follow-up. The investigators expect to find that, after treatment, the group receiving BA will experience higher statistical and clinical significant reductions in depressive symptom severity (primary outcome), as well as in their levels of some pain-related variables (namely pain intensity, fibromyalgia impact, pain-related anxiety, catastrophism, and physical health symptoms severity). In addition, the percentage of women in remission from the diagnosis of depression (as well as the percentage of women responding to treatment) will be greater in the experimental arm than in the comparison arm. Also, the percentage of women who show a clinically significant reduction in pain intensity (decreases greater than two units in the pain intensity scale) will be greater in the experimental arm than in the comparison arm. Finally, the investigators hypothesize that the decrease observed in the pain-related variables will be mediated by the decrease observed in depressive symptoms severity. Regarding the outcomes assessed at a three months follow-up, the investigators expect to find that the differences found after treatment between the two arms will be maintained at follow-up. In order to test our hypothesis, Hierarchical Linear Models (HLM) and Cochran-Mantel-Haenszel tests will be performed. The results of these study might contribute to facilitate the integrated treatment of fibromyalgia and depression, and to reduce the burden on the health system due to the lack of effective therapeutic strategies to treat these comorbidity.