An illegal blindside block is a penalty in football that is called on a player when a blindside block is performed on another, defenseless player. A blindside block occurs when a player blocks an opposing player while the opposing player's body is parallel to that of the other player, making the player unable to see the incoming blocker, and thus being 'defenseless'. This penalty results in a loss of 15 yards for the team whose player committed the foul.
As of May 2019, the NFL outlawed all blindside blocks. Blindside blocks were legal except when the block targeted the neck and head area.