Sea buckthorn leaves (SBT_LVs) form notable by-product during harvesting and post-harvest management of the berries. It is already known that sea buckthorn berries are important for their chemical composition and based on this, they occupy a wide field in nutrition. SBT_LVs also have a rich chemical composition, like the berries. The aim of this study was to describe these by-products in the context of protein and complex carbohydrates–dietary fiber fractions, including qualitative and quantitative composition of amino acids. Proximate composition, amino acids, nutritional values of the protein, and dietary fiber fractions of SBT_LVs of four cultivars (cvs.) Ascola, Habego, Hergo, and Leikora were assessed. SBT_LVs from different years of the study had statistically different levels of crude protein, ether extract, crude ash, and nitrogen-free extract (NFE), confirming that the quality of the raw material (leaves) can be significantly modified by habitat conditions. The largest fraction of dietary fiber was neutral detergent fiber (NDF), including the sum of hemicellulose, cellulose, and lignin, followed by the acid detergent fiber fraction (ADF), consisting of lignin and cellulose. The content of essential amino acids in SBT_LV protein was high. Overall, this study confirms that SBT_LVs hold promise as a valuable resource for use as a food ingredient, functional food, and dietary supplement for both humans and animals.