The aim of the present study was to examine the associations of physical fitness, sedentary time, and physical activity (PA) with working memory and neuroelectric activity in children with overweight/obesity. Seventy-nine children with overweight/obesity (10.2 +/- 1.1 years old) participated in this cross-sectional study. We assessed physical fitness components (ie, muscular strength, speed agility, and cardiorespiratory fitness) using the ALPHA battery. Sedentary time and PA were assessed by GT3X+ accelerometers (ActiGraph). Working memory was assessed using the delayed non-matched-to-sample task; mean reaction time (RT) and response accuracy were registered. Neuroelectric activity (ie, P3 amplitude and latency) was registered using the ActiveTwo System of BioSemi electroencephalogram. Higher upper-limb absolute strength was associated with lower response accuracy (P = 0.023), while higher lower-limb relative-to-weight strength was associated with larger P3 amplitude (P < 0.05). Higher speed agility and cardiorespiratory fitness levels were associated with shorter mean RT and larger P3 amplitude, and speed agility was also associated with shorter P3 latency (all P < 0.05). Vigorous PA was associated with larger P3 amplitude (P < 0.05). No associations were found for sedentary time or the rest of PA intensities (P >= 0.05). In addition to cardiorespiratory fitness, muscular strength and speed agility are also associated with working memory and neuroelectric activity in children with overweight/obesity. The association between PA and working memory is intensity-dependent, as significant findings were only observed for vigorous PA. Randomized controlled trials in this population would help to better understand whether improvements in different components of fitness and PA lead to better working memory and underlying brain function.