This study clinically validated the nursing diagnosis of "sedentary lifestyle" (SL) among 564 Brazilian adolescents. Measures of diagnostic accuracy were calculated for defining characteristics, and Mantel--Haenszel analysis was used to identify related factors. The measures of diagnostic accuracy showed that the following defining characteristics were statistically significant: "average daily physical activity less than recommended for gender and age," "preference for activity low in physical activity," "nonengagement in leisure time physical activities," and "diminished respiratory capacity." An SL showed statistically significant associations with the following related factors: insufficient motivation for physical activity; insufficient interest in physical activity; insufficient resources for physical activity; insufficient social support for physical activity; attitudes, beliefs, and health habits that hinder physical activity; and insufficient confidence for practicing physical exercises. The study highlighted the four defining characteristics and six related factors for making decisions related to SL among adolescents.