Lossl has joined on a season-long agreement from Mainz, with the Terriers able to make the move permanent in 12 months' time.
The capture follows a busy Friday at John Smith's Stadium, with head coach David Wagner having signed an extended contract before the capture of Aaron Mooy - a loan star last term - in a club-record deal from Manchester City.
Wagner's men secured promotion backed up by a loan keeper, having deployed Liverpool's Danny Ward between the sticks for much of 2016-17.
Bringing Ward back appears to have been Wagner's plan, but it was scuppered by Anfield boss Jurgen Klopp.
Wagner said: "I've spoken with Jurgen and unfortunately he's made the decision that Danny Ward has to go back to Liverpool. This is a player we will not be able to bring to our club like we we've done with Aaron."
Lossl has a solitary cap for Denmark and made 27 Bundesliga appearances last season.