"Saido is a player that I feel has got the quality and the finishing power to help us," Cobblers boss Gary Johnson told the club website after landing the 18-year-old England Under-20 international on loan to replace Marlon Jackson.
Jackson's Sixfields spell has ended after his recall by parent club Bristol City.
Berahino showed that he is in good form earlier this week by netting for West Brom's reserves against Arsenal.
"West Brom think very highly of him and I've seen him play a couple of times for their reserves," added Johnson.