The deal made the youngster's 17th birthday week a memorable one and, according to academy boss Richard Stevens, shows how highly the club think of the teen.
Exciting energetic midfielder Whitwell has committed his future to Albion by signing until 2025.
Whitwell, who only signed scholarship forms with the under-18s in the summer, has played in all nine fixtures in Richard Beale's under-21s' Premier League 2 campaign this season against opposition mostly his senior.
Stevens said of the contract: “This is a great reward for the effort and progress Harry has shown since making the step from a school boy to a scholar.
“For a young lad who has just turned 17, he has shown he has the capability of playing regularly in the PL2 team. This shows how highly we rate Harry and I look forward to seeing his continued development at Albion.”
Whitwell, who joined the club's academy as a schoolboy, has caught the eye of supporters who have watched the young Baggies in action at Keys Park and The Hawthorns this term.
Head coach Beale said recently: “He’s going from strength to strength, Harry. He’s had a fantastic year. He’s already trained in the first team under the new manager, he has had several sessions with the first team this season."