A weight adjustable dumbbell comprising a handle assembly, wherein the handle assembly comprises a handle and a first counterweight part and a second counterweight part respectively arranged at two ends of the handle, each of the first counterweight part and the second counterweight part comprises a plurality of dumbbell pieces, each dumbbell piece comprises a mounting hole and an open groove extending from the mounting hole to an edge of the dumbbell piece, a width of the open groove is less than a diameter of the mounting hole; the weight adjustable dumbbell further comprises an adjusting mechanism, the adjusting mechanism comprises a rotating mechanism, a transmission shaft, a first transmission sleeve and a second transmission sleeve, the rotating mechanism is fixedly connected with the transmission shaft to drive the transmission shaft to rotate; an outer diameter of the first transmission sleeve and the second transmission sleeve is greater than the width of the open groove but less than the diameter of the mounting hole, the first transmission sleeve and the second transmission sleeve are sleeved on the transmission shaft, and the transmission shaft is meshed with the first transmission sleeve and the second transmission sleeve, such that the transmission shaft can drive the first transmission sleeve and the second transmission sleeve to move axially along the transmission shaft and cause distal ends of the first transmission sleeve and the second transmission sleeve to insert into or leave the mounting holes of portion or all of the dumbbell pieces, whereby the dumbbell pieces are fixed on or detached from the first counterweight part and the second counterweight part; characterized in that a planetary gear transmission mechanism is used as the rotating mechanism, the rotating mechanism is arranged on an outer side of the first counterweight part, a first cover is provided at an outer side of the rotating mechanism for covering the rotating mechanism; the weight adjustable dumbbell further comprises a prompt mechanism and a self-locking mechanism both arranged on an outer side of the second counterweight part, a second cover is provided at an outer side of the prompt mechanism for covering the prompt mechanism and the self-locking mechanism. the weight adjustable dumbbell further comprises an adjusting mechanism, the adjusting mechanism comprises a rotating mechanism, a transmission shaft, a first transmission sleeve and a second transmission sleeve, the rotating mechanism is fixedly connected with the transmission shaft to drive the transmission shaft to rotate; an outer diameter of the first transmission sleeve and the second transmission sleeve is greater than the width of the open groove but less than the diameter of the mounting hole, the first transmission sleeve and the second transmission sleeve are sleeved on the transmission shaft, and the transmission shaft is meshed with the first transmission sleeve and the second transmission sleeve, such that the transmission shaft can drive the first transmission sleeve and the second transmission sleeve to move axially along the transmission shaft and cause distal ends of the first transmission sleeve and the second transmission sleeve to insert into or leave the mounting holes of portion or all of the dumbbell pieces, whereby the dumbbell pieces are fixed on or detached from the first counterweight part and the second counterweight part; characterized in that a planetary gear transmission mechanism is used as the rotating mechanism, the rotating mechanism is arranged on an outer side of the first counterweight part, a first cover is provided at an outer side of the rotating mechanism for covering the rotating mechanism; the weight adjustable dumbbell further comprises a prompt mechanism and a self-locking mechanism both arranged on an outer side of the second counterweight part, a second cover is provided at an outer side of the prompt mechanism for covering the prompt mechanism and the self-locking mechanism.