Amaranthus cruentus L. (varieties: Juana, Aurelia, Elena) and Amaranthus viridis L. (variety: Callaloo) have long been utilized in food products for human consumption in Central and South America. However, there is limited information on the chemical composition of these species’ leaves and grains grown in Puerto Rico. This study aimed to fill this gap by evaluating the nutritional profile of these four amaranth varieties cultivated in Puerto Rico. A compositional analysis was conducted using official methods, focusing on lysine, protein, dietary fiber, and mineral content. The results showed high lysine content in both species. Significant differences (p < 0.05) were found in crude protein levels among the leaves, with Elena (23%) and Aurelia (21%) showing the highest values. While protein content among grains averaged 19%, there were no significant differences between varieties. The analysis of dietary fiber revealed significant differences (p < 0.05) in insoluble dietary fiber (IDF) and total dietary fiber (TDF) for the leaves and in IDF, soluble dietary fiber (SDF), and TDF for the grains. Additionally, calcium, magnesium, and phosphorus concentrations differed significantly (p < 0.05) in the leaves, while iron, potassium, and zinc showed no significant variation. Significant differences were found in the grains for calcium, magnesium, iron, and phosphorus. This research highlights the excellent nutritional value of amaranth leaves and grains grown in Puerto Rico, with Elena and Aurelia having exceptionally high protein content in their leaves.