An anti-roll bar device with a variable rigidity has a first arm assembly having multiple first joining units, a second arm assembly having multiple second joining units, and a variable rigidity unit mounted between the first arm assembly and the second arm assembly and having multiple abutment portions and a variable rigidity coefficient. The first and second joining units are staggered with each other annularly and abut the abutment portions. When a vehicle passes a bumpy terrain, a slight force is exerted on the variable rigidity unit and is absorbed by the variable rigidity unit, such that the vehicle can be kept from tilting and shaking up and down. When the vehicle is in cornering, a larger force is exerted on the variable rigidity unit to increase a rigidity of the variable rigidity unit, such that the variable rigidity unit can transfer torques to keep the vehicle from tilting.