The invention concerns a spacer for a heat exchanger, said spacer (7) comprising a predefined number of substantially parallel planar walls (13; 113) connected two-by-two by folds (15), the planar walls (13; 113) comprising a plurality of louvers (171; 172) substantially inclined relative to the general plane (P) defined by a planar wall (13; 113). According to the invention, a planar wall (13; 113) further comprises at least one strap (19; 119
1 , 1192, 1193, 1194) having longitudinal sides (21) that extend parallel to the planar wall (13; 113) and lateral sides (23) linked to the planar wall (13; 113) by means of at least one connecting rail (24). The invention also concerns a heat exchanger comprising a heat exchange bundle comprising a stack of parallel tubes (5) for the circulation of the fluid, the tubes having a tube width (l1), and a plurality of such spacers (7) disposed respectively between two tubes (5).