Recently, we have witnessed a rapid increase in the number of research studies in the area of brain fog, predominantly due to the fact that it is reported to be a frequent long COVID condition. However, the construct of brain fog remains ill-defined and a common method of assessment of the condition is lacking. Therefore, the main aim of the current study was to develop and validate a self-report Brain Fog Scale (BFS) for use in clinical and research settings. Participants were 1452 (n = 996, 68.6 % female) Polish university students. The data were collected anonymously through self-completion questionnaires. Results indicate that the 23-item BFS has good psychometric properties. Based on principal component analysis (PCA) and confirmatory factor analysis (CFA) results, the scale is best captured by a three-factor solution, with six items loading on the mental fatigue factor, nine items loading on the impaired cognitive acuity factor, and eight items loading on the confusion factor. We found that individuals who tested positive for COVID-19 had significantly higher mental fatigue, impaired cognitive acuity, and confusion scores than matched controls who never tested positive for COVID-19.