Buckley, 28, will play Clayton – a brash young sports agent who represents Nathan Scott (James Lafferty) as he begins his NBA career on the show.
Former "90210" star Brian Austin Green had reportedly been in talks to take the role.
It marks a return to The CW for Buckley, who previously appeared on the series "Privleged."
While Buckley gets set for his "Tree Hill" debut, the show will be missing two original cast members when the new season premieres, following the departures of Chad Michael Murray and Hilarie Burton.
Austin Nichols, who previously starred in HBO's short-lived "John From Cincinatti," is also joining the cast as a series regular. He made several appearances last season as Julian, a director who wins the heart of Brooke Davis (Sophia Bush).
The new season of "One Tree Hill" is scheduled to debut in September.
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