Following his collegiate career, Castillo turned professional in 2023. He initially earned conditional status on the
Korn Ferry Tour through
PGA Tour University and, in his debut, won the
Blue Cross and Blue Shield of Kansas Wichita Open in June 2023. He later earned a
PGA Tour card for the 2024–25 season. In March 2025, Castillo took the place of
Victor Perez in the
2025 Players Championship after the French golfer withdrew due to a back injury. In March 2026, Castillo won the
Puerto Rico Open for his first PGA Tour title. ==Amateur wins==