Since the end of last season, head coach Tony Pulis has been in the market for a new centre-back and he has been successful in bringing in a player who has previously represented Fiorentina.
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."
The 26-year-old has been a regular with Egypt since 2011, with 39 caps being won for his country during that time.
Hegazi becomes just the second new first-team signing ahead of the new campaign, with attacker Jay Rodriguez having already arrived from Southampton.