Recently, large electrocaloric effects (ECE) in antiferroelectric sol-gel PbZr0.95Ti0.05O3 thin films and in ferroelectric polymer P(VDF-TrFE)55/45 thin films were observed near the ferroelectric Curie temperatures of these materials (495 K and 353 K, respectively). Here a giant ECE (T = 45.3 K and S = 46.9 J K-1 kg-1 at 598 kV cm-1) is obtained in relaxor ferroelectric Pb0.8Ba0.2ZrO3 (PBZ) thin films fabricated on Pt(111)/TiOx/SiO2/Si substrates using a sol-gel method. Nanoscale antiferroelectric (AFE) and ferroelectric (FE) phases coexist at room temperature (290 K) rather than at the Curie temperature (408 K) of the material. The giant ECE in such a system is attributed to the coexistence of AFE and FE phases and a field-induced nanoscale AFE to FE phase transition. The giant ECE of the thin film makes this a promising material for applications in cooling systems near room temperature.