The effect of thermal exposure at 70 and 100 °C on intergranular corrosion (IGC) susceptibility of AA5083 alloy has been studied. At both temperatures, metastable β′ phase has been detected at grain boundaries. The thermal exposure resulted in sensitized microstructure in the alloy and, consequently, increased mass loss values during the nitric acid mass loss test (NAMLT). The sensitization of the alloy is related to the formation of the β′ phase. The as-received O temper alloy showed relatively high resistance to IGC and exhibited a discontinuous distribution of the β′ phase precipitates at grain boundaries. After exposure at 70 °C for 480 h, the size of β′ phase precipitate at the grain boundaries slightly increased, resulting in an increased mass loss. After exposure at 100 °C for 240 h, the alloy became severely sensitized due to the more continuous distribution of β′ phase precipitate along the grain boundaries.