"In order to preserve the integrity of these Games and the safety of all participants, we have decided to refuse the athlete entries from RPC and NPC Belarus," the IPC said in a statement.
The ban was a reversal of a decision the previous day to allow the athletes to compete in the Games, which start on Friday.
The statement said multiple Paralympics Committees, teams and athletes had threatened not to compete if the Russian and Belarusian athletes were allowed, "jeapordizing the viability" of the Games.
"Ensuring the safety and security of athletes is of paramount importance to us and the situation in the athlete villages is escalating and has now become untenable," the statement said.
The International Olympic Committee (IOC) had this week urged sporting federations across the world to exclude athletes from the two countries.
International Paralympic Committee president Andrew Parsons had initially justified Thursday's decision to allow the Russian and Belarusian athletes to compete by saying it was the "harshest possible punishment we can do within the framework of our rules".
Athletes from Ukraine arrived in Beijing on Wednesday.