Defender Lasse Wilhelm joined from FC Saarbrücken on a three-year deal.
Hungarian striker Donát Bárány joined from Debreceni VSC on a four-year contract through 2030.
"Donát was on the radar of several clubs, so we are very pleased that he ultimately chose ADO Den Haag," said Technical Director Mark Wotte.
ADO Den Haag is also pursuing a loan deal for Hoffenheim forward Yannick Eduardo, who previously worked with head trainer Robin Peter during his youth career at RB Leipzig.
Meanwhile, team captain Jari Vlak has departed the club after signing with Spanish side CD Castellón.
"Lasse is a young defender with a good football education and the right mentality to develop further.
We see a lot of potential in him and are convinced that ADO Den Haag is the right environment for him to take the next step in his career," said Mark Wotte.
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ADO Den Haag will continue its preseason schedule with an upcoming friendly match against Belgian club Lommel SK.
