The invention relates to a heat exchanger collector box (3) including: a collector plate (11) having holes respectively lined by collars (19) for passage of ends of heat exchange tubes (7), a sealing means (13), and a cover closing the collector box (3). According to the invention, the sealing means (13) includes at least one support face (29) and is arranged on the collector plate (11) such that: the support face (29) is arranged abutting and under tension against one of the collars (19) when the sealing means (13) is assembled with the collector plate (11), and in the compressed state, the sealing means (13) is kept away from the ends of the heat exchange tubes (7) by interposing collars (19) between the sealing means (13) and the ends of said tubes (7).