We combine classical molecular dynamics simulations and quantum density functional theory calculations to study the temperature effects on the electron affinity of the water octamer. The atomistic simulations provide a sample of the cluster's conformations as a function of the temperature, on which the density functional calculations are carried on. As the temperature increases, the cluster undergoes its characteristic phase change from a cubic, solidlike structure to a liquidlike state. This phase change is also reflected by an increase on the total dipole moment of the cluster. The quantum calculations indicate that the large dipole moment conformations have a positive electron affinity. Relaxing the high temperature conformations of the cluster anion to its local minimum, the average vertical detachment energy is calculated and shows a clear tendency to increase as the temperature increases. The analysis of the high temperatures conformations reveals that origin of higher values of the vertical detachment energy is not the stability of the negative octamer but the high energy of the corresponding neutral cluster.