Bottom ash (BA) dispensed from coal-based thermal power stations has recently gained attention for various geotechnical applications. However, when its utilization is extended to earthquake-prone areas, it is necessary to understand its dynamic characteristics. As the particle size distribution of BA is similar to that of sand, the chances of liquefaction during an earthquake are high. The primary objective of this study was to investigate the effect of the initial conditions, namely the relative density and confining pressure, on the dynamic characteristics of the BA. A set of strain-controlled consolidated undrained cyclic triaxial tests was carried out on BA at various relative densities (25, 50, and 75%) and confining pressures (50, 100, 150, and 200 kPa) with a cyclic shear strain of 0.45% applied at a loading frequency of 1 Hz until the onset of initial liquefaction. The effects of these initial conditions on the undrained cyclic loading behavior of the BA were evaluated in terms of the dynamic shear modulus, damping ratio, and excess pore pressure response. The test results indicated that the dynamic shear modulus and damping ratio of the BA decreased with the progress of the loading cycles for all initial conditions, which can be attributed to the deformation of the BA sample. The dynamic properties of the BA increased when the relative density and effective confining pressure increased, indicating its resistance to liquefaction. Furthermore, the excess pore pressure response indicated a higher liquefaction susceptibility of the BA in the loose state and lower confining pressures. In all cases, the BA could hold some deviator stress at the beginning of cyclic loading. However, with time, its holding capacity decreased to zero owing to the accumulation of excess pore pressure, which created an initial liquefaction in the BA samples.