The 27-year-old joins from Manchester City on a season-long switch, he signed a new four-deal with City last November.
Boro boss Wilder said: “I think it’s a big coup for us and so I’m delighted we’ve brought Zack in.
"We knew we had to improve the goalkeeping department. We brought in Liam Roberts and now we’re adding Zack who has just signed a four-and-a-half-year deal with Man City.
"Man City don’t hand out four-and-a-half-year contracts if they don’t think anything of them.
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“Everyone knows his situation, he needed to play because of the World Cup.
"He had an agreement with Man City, but he could have easily stayed there and sat comfortably as their number two. He wanted to go out and he’s agreed to come here.”
Steffen has 29 caps for his country and has represented the USA at all levels from Under-18.