The invention concerns a tank for a fluid product, having a bottom (4) and a resistant side wall (10) composed of juxtaposed prefabricated panels (1) each comprising a wall element (2) covered, on an internal face (22), with a continuous sealing sheet (3) extended by three sections (33, 33′, 34) which cover respectively the lateral sides (23, 23′) and the lower side (24) of the wall element (2) and overlap on the outside of the wall (10). Said panels are placed on a slab (4) covered with a sealing sheet (42) and the adjacent overlapping sections of the sealing sheets (3, 42) covering the set of the panels (1) and the bottom (4) are connected together in a sealed fashion.Such a tank which is easy to build and to maintain, can contain drinking water or a polluting product.