Motivated by the seismic response of unanchored liquid storage tanks, their uplift mechanism under strong lateral loading is examined. Using three-dimensional finite element models, nonlinear static analysis is conducted to define the moment-rotation relationship of uplifting tanks resting on rigid foundation and describe the evolution of critical response parameters with increasing level of lateral loading. Meridional and hoop stress, as well as their distribution and evolution with increased uplift are computed. Comparing the numerical results with the corresponding results from anchored tanks, striking differences are observed on the values of compressive meridional stresses and their distribution around the tank circumference. Cyclic analysis, associated with repeated uplift at both sides of the tank, is also performed, to obtain the corresponding hysteretic response and verify the assumption of nonlinear-elastic tank behaviour, used in several previous works. Finally, an analytical solution is developed, capable of describing tank uplift in an efficient manner. The analytical solution accounts for the special features of uplift, obtained from the finite element solution, and can be used for simple and reliable assessment of seismic performance in unanchored liquid storage tanks.