Methods, systems, and devices for spare substitution in a memory system are described. A controller may, as part of a background operation, assign a spare bit to replace a bit of a code word and save an indication of the spare bit assignment in a memory array. The code word may include a set of bits that each correspond to a respective Minimum Substitution Region (MSR) within a memory medium that retains the code word. An MSR corresponding to the bit to be replaced may include a quantity of erroneous bits relative to a threshold. The controller may, during a read operation, identify the spare bit in a first portion of the code word, determine the bit to be replaced based on accessing the memory array, and replace the bit with the spare bit concurrently with receiving a second portion of the code word.