Background/Objectives: Rhythmic gymnastics combines high training demands with aesthetic pressure. This pilot study assessed dietary intake, body composition, and nutrition-related behaviours in Czech competitive rhythmic gymnasts. Methods: Fourteen female athletes (juniors aged 13–15 years, n = 6; seniors aged 16–20 years, n = 8) completed a three-day weighed food record at the baseline and underwent multifrequency bioelectrical impedance assessment at the baseline and approximately three months later, following individualised nutrition advice. A separate online questionnaire was completed by 50 gymnasts. Results: Reported energy intake averaged 63.7% and 67.1% of the estimated requirements among juniors and seniors, respectively. Carbohydrate, fibre, vitamin D, calcium, magnesium, and potassium intakes were low relative to prespecified study benchmarks. No statistically significant within-group changes in body composition were observed. An exploratory between-group difference in changes in the extracellular-to-total body water ratio was found (unadjusted p = 0.015), requiring cautious interpretation. Among the questionnaire respondents, 68% reported weight-related pressure, 60% intentional food restriction, and 56% had never heard of relative energy deficiency in sport (RED-S). Conclusions: Low reported intakes and weight-related concerns suggest potential need for qualified nutritional support. Intakes below the prespecified benchmarks do not establish individual nutritional inadequacy. The small uncontrolled cohort and absence of follow-up dietary records preclude conclusions about the effectiveness of nutritional counselling, while the short dietary assessment and age-independent benchmarks limit individual dietary interpretation and do not permit the diagnosis of clinical nutrient deficiencies, low energy availability, or RED-S.