An anti-slosh device for attenuating oscillations of liquids in tanks caused by movement of vehicles in which the tank is located comprises an elongated flat, rectangularly, shaped, uniform thickness strip of a liquid impervious material such as polyethylene or polycarbonate which is formed into a string of arcuately curved loops. One embodiment of the device has located at opposite ends thereof spirally-shaped, quasi-helical end loops which form a figure-8 shape, and a center loop disposed obliquely between the end loops. A two-loop version of the device has two end loops but no center loop. The anti-slosh device is used to reduce the duration of undesirable oscillations induced in a liquid in a tank by placing a plurality of the devices in the interior of a tank, as by inserting a plurality of the devices through a fill tube or other access port of the tank. Non-parallel surfaces of the device function as baffles, converting large scale linear motions of liquid in a tank to small scale, energy-dissipating vortices, thus reducing the duration and amplitude of oscillations of liquid in the tank.