The 18-year-old joins at the Premier League club from Finnish top flight outfit Ilves Tampere.
Boro's announcement did not disclosed the transfer fee, although reports claim he was signed for around £170,000.
Soisalo, who is considered one of Finland’s brightest young talents, scored seven league goals and claimed six assists last season and will initially play with Boro’s Under-23s team.
The starlet was also voted in the Veikkausliiga team of the season last year, and has been capped by his country at Under-19 level.