Pruning previously-allocated free blocks from a synthetic backup. In one example embodiment, a method of pruning previously-allocated free blocks from a synthetic backup includes identifying multiple sequential backups to be included in a synthetic backup, accessing a copy of a file system block allocation map (FSBAM) of the most recent of the multiple sequential backups that was stored with the most recent backup, identifying a set of blocks that includes the most recent block for each unique block position contained within the multiple sequential backups, pruning the set of blocks to exclude all blocks that are identified as being free in the FSBAM, storing the pruned set of blocks in the synthetic backup, and storing a copy of the FSBAM with the synthetic backup.