AbstractFibromyalgia syndrome (FMS) is a complex pain disorder, characterized by diffuse pain and cognitive disturbances. Abnormal cortical oscillatory activity may be a promising biomarker, encouraging non-invasive neurostimulation techniques as a treatment. We aimed to modulate abnormal slow cortical oscillations by delivering transcranial alternating current stimulation (tACS) and physiotherapy to reduce pain and cognitive symptoms. This was a double-blinded, randomized, crossover trial conducted between February and September 2018 at the Rehabilitation Unit of a teaching Hospital (NCT03221413). Participants were randomly assigned to tACS or random noise stimulation (RNS), 5 days/week for 2 weeks followed by ad hoc physiotherapy. Clinical and cognitive assessments were performed at T₀ (baseline), T₁ (after stimulation), T₂ (1 month after stimulation). Electroencephalogram (EEG) spectral topographies recorded from 15 participants confirmed slow-rhythm prevalence and provided tACS tailored stimulation parameters and electrode sites. Following tACS, EEG alpha1 ([8–10] Hz) activity increased at T₁ (p = 0.024) compared to RNS, pain symptoms assessed by Visual Analog Scale decreased at T₁ (T₁ vs T₀p = 0.010), self-reported cognitive skills and neuropsychological scores improved both at T₁ and T₂ (Patient-Reported Outcomes in Cognitive Impairment, T₀–T₂, p = 0.024; Everyday memory questionnaire, T₁ compared to RNS, p = 0.012; Montréal Cognitive Assessment, T₀ vs T₁, p = 0.048 and T₀ vs T₂, p = 0.009; Trail Making Test B T₀–T₂, p = 0.034). Psychopathological scales and other neuropsychological scores (Trail Making Test-A; Total Phonemic Fluency; Hopkins Verbal Learning Test-Revised; Rey–Osterrieth Complex Figure) improved both after tACS and RNS but earlier improvements (T₁) were registered only after tACS. These results support tACS coupled with physiotherapy in treating FMS cognitive symptoms, pain and subclinical psychopathology.