Salah, who has not been at his best since returning from injury last month, was brought on in the 79th minute, just after the Hammers' second goal levelled the game
Luis Suarez had put the hosts in charge in the 15th minute, producing an ice-cool finish to beat Thibaut Courtois after a lung-bursting run forward from his own half by midfielder Marcos Llorente.
An Atletico victory would have taken Diego Simeone's side eight points clear of their city rivals but Benzema's leveller means they lead third-placed Real by five points while Barcelona, in second, are three points behind.