Background: Previous studies have reported good test–retest reliability for the 3 m backward test (3MBWT) in different populations. However, reliability of the 3MBWT has not been studied in fibromyalgia (FM) under single and dual-task conditions; Methods: A total of 21 women with FM participated in this study. Participants completed the Revised Fibromyalgia Impact Questionnaire and two physical fitness tests: the 3MBWT and the Timed Up and Go (TUG). The dual-task condition consisted of subtracting two by two while performing the test, starting from a random number less than 100; Results: Values showed that the 3MBWT can be considered reliable under single and dual-task conditions when measured with both a manual stopwatch and a Chronopic automatic stopwatch. A strong concurrent validity was shown of 3MBWT and TUG results in the test and retest and the different devices. The relationship between the performance of the 3MBWT in test and retest conditions under single and dual-task conditions measured with different devices and the impact of the disease were high; Conclusions: The 3MBWT is a reliable tool under the single and dual-task conditions in women with FM. It shows higher reliability values when time is taken using a Chronopic. This test also shows high concurrent validity with the TUG test. Its performance is related to the impact of the disease.