The defender who has been capped 39 times for the Pharaohs' ventures into his second stint in European football following an unsuccessful stint with Fiorentina which was plagued with recurring injuries.
''We're delighted to add Ahmed to the squad.’’ Albion chairman John Williams told club website.
''He caught our eye at the African championships and we've been tracking him ever since."
In his own remark, manager Tony Pulis is pleased to have the 26-year-old in his fold and feels that is good for the Hawthorns giants.
"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."
Hegazy played a crucial role for the Pharaohs who finished as runners-up at the 2017 Africa Cup of Nations.
There, he was named in the Caf Team of the tournament alongside compatriot Mohamed Salah.
West Brom begin next season's campaign with a home tie against Bournemouth on August 12.