OBJECTIVE: This observational study examined the feasibility, reliability, and validity of repeated ambulatory cognitive tests in fibromyalgia (FM). METHOD: Adults with FM ( = 50) and matched controls ( = 50) completed lab-based neuropsychological tests (NIH Toolbox) followed by eight days of smartphone-based ambulatory testing of processing speed (symbol search) and working memory (dot memory) five times daily. Feasibility was assessed based on response rates. Reliability was evaluated using overall average between-person reliabilities for the full assessment period and by determining the number of assessment days necessary to attain reliabilities of >.80 and >.90. To assess convergent validity, correlations were calculated between ambulatory test scores and NIH Toolbox scores. Test performance was contrasted between the FM and non-FM groups to examine known-groups validity. RESULTS: Average rates of response to the ambulatory cognitive tests were 89.5% in FM and 90.0% in non-FM. Overall average between-person reliabilities were ≥.96. In FM, between-person reliability exceeded .90 after two days for symbol search and three days for dot memory. Symbol search scores correlated with NIH Toolbox processing speed scores in both groups, though there were no significant group differences in symbol search performance. Dot memory scores correlated with NIH Toolbox working memory scores in both groups. FM participants exhibited worse dot memory performance than did non-FM participants. CONCLUSIONS: Repeated ambulatory tests of processing speed and working memory demonstrate feasibility and reliability in FM, though evidence for construct validity is mixed. The findings demonstrate promise for future research and clinical applications of this approach to assessing cognition in FM.