The 25-year-old midfielder earned his move to the Hawthorns after impressing on trial in December.
West Brom manager Tony Mowbray told the club's official website: "Do-Heon has undoubted pedigree.
"He is another player we have assessed during a training spell. He showed us during that time he has the ability and talent to create further competition within our squad.
"There may be a period in which Do-Heon needs to acclimatise to the British game because this is his first time in Europe.
"But we're hoping that will be a short period."