Alex Gilbert is Boro's first summer acquisition, arriving at the end of his contract with Brentford.
The forward, 21, signs an four-year deal at the Riverside.
A Republic of Ireland youth international, Alex captained Brentford's B side in 2022/23 as they lifted the Premier League Cup.
Comfortable across the frontline, he scored 16 goals and provided eight assists in his most recent campaign, and was named Brentford B's Player of the Year.
He joins Michael Carrick's Boro after declining new terms at Brentford amid reported interest from other top-flight clubs.
Alex has already flown out to Portugal to join Boro's training camp in the Algarve.