The objective of this study was to validate the defining characteristics for the nursing diagnosis of Fatigue in adult oncological patients. It is a cross-sectional, descriptive study with a quantitative perspective, and its type is diagnostic content validation. Data collection was carried out in a University Hospital. The sample was made up of 35 expert nurses. The instrument used was subdivided into four parts. The data was analyzed by descriptive statistics. 15 defining characteristics were identified, considered secondary indicators. With an average weighting of less than 0.50, four defining characteristics were excluded. The defining characteristic Impaired social interaction, added to those described by NANDA-I after review of the literature, was validated with a weighted average of 0.71. It was concluded that the subjectivity of the defining characteristics and the difficulty nurses have in recognizing them influence the identification of this diagnosis.