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Using Simple Ocean Data Assimilation data set for 1951–2010, we analyze the decadal variability of barrier layer thickness (BLT) and its forcing mechanisms in the Bay of Bengal (BoB). The decadal variability of BLT shows a close connection with the Pacific Decadal Oscillation (PDO). The PDO‐induced spatial distribution of BLT is similar to its first Empirical Orthogonal Function mode, showing a meridional dipolar pattern. PDO can exert an influence on the decadal variations in the Walker circulation to indirectly modulate decadal variations in BLT. Weakened southwesterly winds over the BoB associated with a positive PDO phase enhance shoreward Ekman transport along the eastern and northern boundary of the BoB and therefore deepen isothermal layer depth, which further induces positive BLT anomalies along the eastern and northern coast of the BoB. On the other hand, the weakened winds decrease oceanic heat loss; meanwhile, the enhanced upwelling driven by positive wind stress curl anomalies shoals the isothermal layer depth and thus decreases the BLT over the southeastern BoB. The opposite is true for the negative PDO phase. Using an ocean mixed layer model, we diagnose the relative contribution of different forcing terms to BLT variations on decadal timescales. It is found that horizontal advection and freshwater flux‐induced entrainment dominate the decadal BLT variability in the northern BoB, whereas heat and freshwater flux‐induced entrainment play a dominant role in the southern BoB. Key Points: Based on long‐term reanalysis data, the decadal variations of barrier layer thickness in the Bay of Bengal are examinedPacific Decadal Oscillation can indirectly modulate the barrier layer thickness variability of Bay of Bengal on the decadal timescalesThe dynamical mechanisms of decadal barrier layer variations are different in the north and south basin [ABSTRACT FROM AUTHOR]