The aim of this study was to analyze the effects of a 24 week exergame intervention and 24 weeks of detraining on lower-limb strength, agility, and cardiorespiratory fitness in women with fibromyalgia (FM). It was performed as a single-blinded randomized controlled trial of 55 women with FM. University facilities were used. The 24 week exergame intervention was focused on mobility, postural control, upper- and lower-limb coordination, aerobic fitness, and strength. Participants performed 120 min of exergaming per week, which was divided into two sessions. Twenty-four weeks after the end of the intervention, participants were re-evaluated. A chair–stand test, 10 step stair test, and six-minute walk test were conducted to assess lower-body strength, agility, and cardiorespiratory fitness, respectively. The exergame intervention significantly improved lower-limb strength and cardiorespiratory fitness. However, no significant effects on agility were observed. After the detraining period, lower-limb strength and agility returned to their baseline level, but improvements in cardiorespiratory fitness were sustained over time. Exergaming was therefore shown to be beneficial for physical fitness in people with FM. However, exergames had to be played regularly to maintain the benefits. This long-term intervention (24 weeks) may have changed the lifestyle of women with FM, which could explain why cardiorespiratory fitness improvements remained after the detraining period. Future research should focus on lifestyle changes after long-term interventions.