USA international Matthew Hoppe has joined the club from La Liga club Mallorca.
The 21-year-old has signed a four-year contract with the Boro.
Hoppe, whose career started with the LA Galaxy Academy, has had spells in Europe with Bundesliga outfit Schalke 04 and Mallorca.
He scored six goals in the Bundesliga and became the first American to score a hat-trick in the German top division.
Hoppe made his international debut against Martinique in July 2021 and scored his first USA goal soon after in a win against Jamaica in the CONCACAF Gold Cup quarter-finals.
Hoppe is the second striker to join the club this summer, following in the footsteps of Marcus Forss, who scored his first goal last weekend.
The California-born marksman is also the second USA international to join the club with goalkeeper Zack Steffen arriving last month on a season-long loan deal from Manchester City.