The invention relates to a composite sandwich panel (1) comprising at least a web (2) provided between an inner skin (3) and an outer skin (4), each skin being made of a composite material from a plurality of fibrous plies (5, 6, 7, 8), characterized in that a portion of the plies of the inner skin and/or of the outer skin each extends at least partially through the web at an uninterrupted area thereof in order to form together a through-reinforcement (9) extending from the inner skin to the outer skin of the composite sandwich panel. The invention further relates to an aircraft element including such a composite sandwich panel.