Objective The aim of the present study is to investigate the associations between total energy, macronutrient intakes, and physical activity (PA) and body composition by assessing body fat mass, fat‐free mass, and BMI in a population of young adults. Methods The study population consisted of 605 young Spanish adults (median age 20.38 ± 2.67). Body composition, including fat mass and fat‐free mass, was calculated with body composition analyzer. Daily energy and macronutrient intakes were measured using a 72‐h recall method. The International PA Questionnaire was used to assess PA and sedentary time. Linear regression analyses were performed to test the possible associations between nutrition, PA factors, and body composition. Results Linear regression analyses revealed that BMI has a significant positive association with protein intake (P = .004, B = 0.088, 95% CI 0.028–0.149) and an inverse association with carbohydrate intake (P = 0.034, B = −0.027, 95% CI −0.053 – −0.002). Protein intake also demonstrated a significant association with fat‐free mass, but the size of the effect was smaller (P = .027, B = 96.965, 95% CI 11.250–182.679). There was evidence of a positive association between total PA and moderate‐to‐vigorous PA (P < .001, B = 15.630, 95% CI 6.989 −24.270) and fat‐free mass (P < .001, B = 20.208, 95% CI 9.694 −30.723). When fat mass was used as the outcome variable, there was no evidence of any association with the PA, total energy, and macronutrient intakes variables analyzed. Conclusions Our findings suggest that PA variables were consistently associated with body composition, specifically fat‐free mass. Dietary factors also have influence over body composition; we showed that protein intake is significantly associated with fat‐free mass and BMI.