The 20-year-old will link up with Mark Warburton’s side ahead of the new Championship season, after spending the second half of the last campaign on loan at NK Maribor.
Technical director Dan Ashworth said: “Jan is one of a number of exciting young players that we have at the club, and this loan move is an excellent next step in his development.
“This allows him to gain a first taste of English football, in what is a very competitive Championship division.
“Myself, Graham, David Weir and his team will keep a very close eye on his progress, and wish him all the best for the upcoming season.”
Mlakar arrived at Albion in January from Maribor, before being loaned back there for the remainder of the 2018/19 season, and was part of the squad that won the Slovenian title.
Elsewhere, the striker has played in Italy for Fiorentina, and spent a brief spell on loan at Serie B side, Venezia.
He has represented Slovenia from under-16 up to under-21 level, where he has scored six goals in 15 appearances.