The 21-year-old has joined the League Two side on a deal that runs until January, subject to EFL approval.
Sivi’s arrival comes after Harrogate boss Simon Weaver’s admission that no more transfer business was anticipated at the EnviroVent Stadium.
Harrogate’s latest addition began his career with Leyton Orient before joining Middlesbrough in 2021.
He spent last season featuring for Middlesbrough’s under-21 side, notching five goals in 12 appearances.
Weaver said: “He’s a tall, wide forward that is blessed with pace and trickery and can score goals, so we’re really excited about bringing him in.
"That vitality that these lads seem to bring with a point to prove coming into senior football, he has bags of talent and athleticism, hopefully we can provide him with a stage to showcase his talents.
“He’s going to be surrounded by good people at the club that want to help him, we think he’ll relish playing for points and get excited about playing men’s football where the pressure is on and the crowds are loud.”