A key part of the Glory's 2018-19 A-League premiership after signing from Croatian club Rijeka last August, Davidson made 28 appearances last season, scoring two goals and notching five assists.

It's understood the interest from Ulsan developed quickly, with an offer accepted within a week of the K-League team approaching the Glory.

Glory chief executive Tony Pignata said the sale had been made reluctantly but there was little the club could do after Davidson invoked a clause in his contract.

"As a club, we insisted at all times that Jason was a required player," Pignata said in a statement.

'We do, however, understand his need to safeguard his family's financial future and his move to the K-League will undoubtedly contribute significantly to doing that."

Ulsan won the 2012 Asian Champions League and are firmly in contention for this year's K-League title, sitting second on the ladder after 16 matches.

"In football when opportunities arise like this you need to grab them with both hands," Davidson told The World Game website.

"Ulsan are a big club and are everything that Perth - having qualified for the (AFC) Champions League - are striving to follow."

A member of Australia's 2015 Asian Cup winning squad, Davidson is yet to feature for the Socceroos during Graham Arnold's reign.

"Having an opportunity like this, to be able to go over there to a big club, and hopefully play week-in week-out will give me a stronger claim to the national team," said Davidson, who has made 22 appearances for the Socceroos.

"I must go over there and perform and get the job done like I did when I came to Perth. The hard work begins now."

Australian Associated Press