Background/Objectives: The goals of sports nutrition are to support athletic performance. However, the diet quality of athletes remains poorly understood. This scoping review aims to synthesise the existing body of literature, evaluating the diet quality of adult athletes. Methods: The scoping review was conducted in accordance with guidance from the Joanna Briggs Institute. Database searches were conducted in Medline, Scopus, SPORTDiscus, Web of Science, and EMBASE. Eligible studies were observational, utilised a validated dietary quality assessment tool (diet quality index), and assessed athletes aged 18 to 65 years. Screening was performed independently by two researchers, with any conflicts resolved by a third researcher. Results: The search yielded 1142 articles, of which 18 met the inclusion criteria. Basketball athletes and gymnasts were the most frequently examined groups. Studies were dominated by one gender (n = 7 all males studies, n = 4 only female). Eleven diet quality tools were used, including variations of the Healthy Eating Index (n = 7). Overall diet quality was rated as poor (n = 3 studies), needs improvement (n = 12), or adequate (n = 3). Food groups where intake was suboptimal included wholegrains (n = 8), fruit (n = 5), and dairy intake (n = 3). The intake of protein-rich foods was adequate in n = 9 studies, whilst fat intake was excessive in 4 studies. Conclusions: These findings suggested that the diet quality of athletes is suboptimal and needs improvement in the areas of wholegrain, fruit, and dairy/alternatives intake. The inadequate intake of these food groups and the consumption of excessive fat intake may increase the long-term risk of developing non-communicable diseases and impair short-term performance and recovery. Further exploration is warranted to develop targeted nutrition education and interventions that address these inadequacies.