An adaptive sliding block Viterbi decoder (ASBVD) includes forward and backward Viterbi processors, a state estimator and a control unit. The processors generate metrics of states and of transitions between the states associated with an encoder, based on encoded input information symbols received via a communications channel. Each processor includes a plurality of buffers for storing information symbols so that a number of the encoded input information symbols can be concurrently decoded. The state estimator estimates a current state of a code trellis based on the generated metrics, and the processors decode the stored information symbols based on the estimated current state. The control unit adapts the number of encoded input information symbols to be concurrently decoded based on a condition of the communications channel, and selectively controls the number of buffers that are enabled in accordance with the number of encoded input information symbols to be concurrently decoded.