Background: Ketogenic diet (KD) is increasingly recognized as a strategy to combat obesity. However, its effects on cognition in sedentary healthy young adults remain underexplored. Methods: In a quasiexperimental design, 186 participants were screened, 78 excluded based on predefined criteria, leaving 108 healthy adults (age 25-45 years, BMI 18-29.9 kg m-2, sedentary <5,000 steps/day) assigned to either KD group (<5% carbohydrates, 20-25% protein, 70-75% fat; n - 54) or control group (regular diet ~50-65% carbohydrates; n - 54). Participants underwent a 4 weeks' dietary intervention. Cognitive domains were assessed at baseline and post intervention using validated computer-based test battery. Pre, mid and post weight, BMI, blood ketones and fasting glucose were also measured. Forty-three participants in the KD group and 38 in the control group completed the trial. Results: Four weeks of non-energy restricted KD improved processing speed, semantic memory, working memory, episodic memory, fluid cognition, crystallized cognition and overall cognitive composite scores (all P <= 0.001) versus controls. Attention and inhibitory control (P - 0.46) and cognitive flexibility (P 5 0.21) showed no significant differences. Blood ketones increased (0.12-1.32mmol L-1, P < 0.001) in KD participants versus controls (0.118-0.105mmol L-1, P - 0.94). KD reduced weight (P < 0.001), BMI (P < 0.001) and fasting glucose (P < 0.001). Post intervention ketones predicted cognitive gain in most cognitive domains except attention & inhibitory control and cognitive flexibility. Conclusion: Short term KD intervention enhances memory, processing speed, fluid, crystallized and overall cognitive function composite scores in sedentary healthy adults.  Kulcsszavak: ketogenic diet, cognition, ketosis, working memory, episodic memory, attention