According to Ruhr Nachrichten, Gardarsson will move to Dortmund from UMF Stjarnan Gardabaer once the paperwork is finalised, with an official announcement to follow. The Iceland Under-17 international stands at 1.94m, is left-footed and made his senior debut for Stjarnan in May, going on to make two further brief appearances for the first team, totalling 31 minutes.
Gardarsson will initially join Dortmund's Under-19s, working under coach Felix Hirschnagl, in a squad in need of a reliable focal point in attack. Beyond his physical presence, sporting director Ole Book and the club's recruitment staff are said to have been particularly impressed by his intelligence, technique and willingness to learn. He has represented Iceland eight times at Under-17 level, scoring twice.
The move follows Dortmund's confirmed signing of defender Liam Claude Kante, 16, from Croatian club NK Lokomotiva Zagreb, and comes shortly after the club also added Japanese Under-19 international Takato Yamamoto on a season-long loan from Gamba Osaka for their Under-23 side.
Together, the three signings underline Dortmund's continued strategy of identifying promising teenage talent from across the world at an early stage, in the hope that at least some of them can eventually break into the first team.
Gardarsson is understood to have already visited Dortmund back in February to familiarise himself with the club's facilities ahead of a potential move, with the transfer having been in the works for several months before now edging closer to completion.
He is set to succeed Taycan Etcibasi as the focal point of Dortmund's Under-19 attack, with the academy graduate having instead opted to join Bundesliga 2 side Hannover 96 a year before his contract was due to expire.
Scouts who have watched Gardarsson describe him as a physically robust number nine with a strong left foot, good technique and surprising pace for his size, qualities that mark him out as one of the more promising attacking prospects to emerge from northern Europe in his age group.