Lightweight washtub structure for washing machine of the type used in automatic washing machines as static support for the rotary drum, maintaining appropriate leaktightness, characterized by an alveolar structure formed from injected thermoplastic material, which includes, in said material, a large quantity of diminutive hollows of spherical or quasi-spherical shape and of random distribution, giving rise to a lightening of the weight of the washtub without detriment to its mechanical properties or leaktightness. This structure affords a number of advantages over devices currently available, the most significant being the fact that it is possible to produce it with a smaller quantity of thermoplastic material, by virtue of its lightened structure, thereby achieving a lower economic cost that makes it more competitive without detriment to its structural performance levels.