The 26-year-old centre-back moves to the Baggies from Egypt, where he has spent the last two years following his transfer from Fiorentina.
Hegazy played 18 games for Al Ahly last season in all competitions, scoring one goal in the CAF Champions League.
Baggies manager Tony Pulis told the club's official website: "We're pleased he's here, he's a good addition. He's got good pedigree and has played for some good clubs.
"Hopefully he'll be a good signing for us. With it being a loan and, if he does well, we've already agreed a final deal. It's really, really good for us."
Albion chairman John Williams added: "We're delighted to add Ahmed to the squad. He caught our eye at the African championships and we've been tracking him ever since."