A snap fastener, of the type comprising a female part (2) including a spring (11, 11a, 11b) and a male part (4) including a stem (5) for engagement with the spring, wherein a chamber (10, 10a, 10b) is provided for housing the spring and including shoulders (12, 12a, 12b) on a buttoning side, the shoulders having a length (L1) longer than a length (L2) of shoulders (13, 13a, 13b) arranged on the opposite side. Thus, a differentiated effort buttoning and unbuttoning mechanism between the stem (5) and the spring (11) is achieved. With respect to prior snap fasteners, the inventive snap fastener provides the advantage of requiring very small buttoning efforts, much smaller than those necessary for unbuttoning the snap fastener, thereby facilitating the snap fastener closing operation while providing a tightness closure upon having buttoned the snap fastener male and female parts.