Reliable and valid assessment of sports nutrition knowledge can inform athlete nutrition education to address knowledge gaps. This study aimed to test the reliability and validity of an electronically administered sports nutrition knowledge tool - Platform to Evaluate Athlete Knowledge of Sports Nutrition Questionnaire (PEAKS-NQ). A 94-item PEAKS-NQ was piloted to 149 developmental athletes (DA) in New Zealand, with a subset invited to complete the PEAKS-NQ again to assess reliability. Reliability was evaluated using sign test, intraclass correlation and Cronbach's α. Accredited sports dietitians (ASD; n 255) completed the PEAKS-NQ to establish construct validity via known-groups methodology and provided relevance scores to determine the scale content validity index (S-CVI). Rasch analysis was conducted to identify potentially problematic items and test reliability. Score differences between DA and ASD were analysed using independent t or non-parametric tests. DA (n 88) were 17·8 (sd 1·4) years, 61·4 % female and mostly in high school (94·3 %). ASD (n 45) were 37·8 (sd 7·6) years, 82·2 % female, with >5 years of dietetic experience (59·1 %). ASD scored higher than DA in all sections and overall (91·5 (sd 3·4) v. 67·1 (sd 10·5) %) (P < 0·001). There were no differences between retests (n 18; P = 0·14). Cronbach's α was 0·86. S-CVI indicated good content validity (0·88). Rasch analysis resulted in a fifty-item PEAKS-NQ with high item (0·91) and person (0·92) reliability. The PEAKS-NQ is reliable and valid for assessing sports nutrition knowledge which could assist practitioners effectively tailor and evaluate nutrition education.