This study examined whether sluggish cognitive tempo (SCT) and attention-deficit/hyperactivity disorder (ADHD) inattention (IN) symptoms demonstrated cross-setting invariance and unique associations with symptom and impairment dimensions across settings (i.e., home SCT and ADHD-IN uniquely predicting school symptom and impairment dimensions, and vice versa). Mothers, fathers, primary teachers, and secondary teachers rated SCT, ADHD-IN, ADHD-hyperactivity/impulsivity (HI), oppositional defiant disorder (ODD), anxiety, depression, academic impairment, social impairment, and peer rejection dimensions for 585 Spanish 3rd-grade children (53% boys). Within-setting (i.e., mothers, fathers; primary, secondary teachers) and cross-settings (i.e., home, school) invariance was found for both SCT and ADHD-IN. From home to school, higher levels of home SCT predicted lower levels of school ADHD-HI and higher levels of school academic impairment after controlling for home ADHD-IN, whereas higher levels of home ADHD-IN predicted higher levels of school ADHD-HI, ODD, anxiety, depression, academic impairment, and peer rejection after controlling for home SCT. From school to home, higher levels of school SCT predicted lower levels of home ADHD-HI and ODD and higher levels of home anxiety, depression, academic impairment, and social impairment after controlling for school ADHD-IN, whereas higher levels of school ADHD-IN predicted higher levels of home ADHD-HI, ODD, and academic impairment after controlling for school SCT. Although SCT at home and school was able to uniquely predict symptom and impairment dimensions in the other setting, SCT at school was a better predictor than ADHD-IN at school of psychopathology and impairment at home. Findings provide additional support for SCT's validity relative to ADHD-IN. (PsycINFO Database Record