'Sagan-Ruby' is the first grapefruit (Citrus paradisi) cultivar to be developed in Japan and is used for food, cosmetics, and other purposes owing to its favorable characteristics, such as the absence of harmful pesticides and its long shelf life. The desired qualities of grapefruit depend on the specific use, and these qualities are influenced by the metabolite composition of the fruits. However, little is known about the influence of the growing environment or harvest period on the metabolite composition of the 'Sagan-Ruby' grapefruit. Therefore, we harvested fruits that were grown either in a plastic house without artificial heating or outdoors with rain cover from December, 2014 to April, 2015, on a monthly basis, and we investigated the composition of the primary metabolites such as sugars, organic acids, and amino acids, in the juice and peel of the fruit using gas chromatography mass spectrometry (GC/MS). We detected a total of 53 and 68 compounds in the juice and peel, respectively, and the first and second components of the principal component analyses of the detected metabolites of both juice and peel were associated with the growing environment and harvest period, respectively. Since we observed that glucose, fructose, sucrose, and citric acid were more concentrated in the juice of outdoor-grown fruits than in that of the house-grown fruits, especially in March and April, it is likely that the sweetness and acidity of the fruits are dependent on the growing environment. Similarly, the primary metabolite contents, including succinic acid and other organic acids, were higher in peels from outdoor-grown fruits. In addition, we also observed that the contents of proline, phenylalanine, and other amino acids in the juice increased continuously from December to April, and many sugars, including glucose and fructose, gradually decreased in peels from December to February and were lower from February to April. These results indicated that quality of the 'Sagan-Ruby' grapefruit varies with the harvest period.