The objective of this study is to test the efficacy of a group motivational plus implementation intentions intervention in promoting adherence to an unsupervised walking program recommended for fibromyalgia, compared to an implementation intentions condition and to an active control condition. A triple-blind, randomized, longitudinal study with measures at baseline, short (seven weeks post-intervention), mid (12 weeks) and long-term (36 weeks) is performed. Data are analyzed using multilevel longitudinal growth curve two-level modelling. Participants are 157 women with fibromyalgia. In the short-term, adherence to the minimum and to the standard walking program (primary outcome measures) is explained by time (both <.001), motivational plus implementation intentions intervention (both <.001) and by their interaction (both <.001). Regarding the secondary outcomes, only physical function is explained by time ( <.001), motivational plus implementation intentions intervention ( <.05) and by their interaction ( <.05). Motivational plus implementation intentions intervention achieve the promotion of walking as an exercise in the short-term; furthermore, physical function of the women in this condition is better than in the other two intervention groups, which is a relevant outcome from a rehabilitation point of view. However, more studies are needed to maintain the exercise at mid and long-term.