Birthplace: Seoul, South Korea
Date of Birth: 21/02/1991
Nationality: South Korean
Previous clubs: INAC Kobe Leonessa
Ji So-Yun joined the club from Japanese league and cup holders INAC Kobe Leonessa, who defeated Chelsea Ladies in the final of December's International Women's Club Championship (IWCC) in Tokyo.
The three-time Korean Footballer of the Year was Emma Hayes's fifth signing this winter, having already added England quartet Rachel Williams, Laura Bassett, Gilly Flaherty and Katie Chapman to her squad for this year's Women's Super League, which gets underway in April.
'What a fantastic signing for the club,' said Hayes. 'I believe she is one of the best midfielders in the world and our fans will love her.'
Ji, who will become the first Korean woman to play in the WSL, added: 'I am glad to be joining Chelsea Ladies and look forward to the new challenge of playing football in Europe.
'I will get myself in gear to become an integral player for my team and to lead Chelsea to the highest possible levels.
'I hope many fans will be greatly interested in our team and my activities. I will always do my best for our club and Chelsea fans.'
Hayes was delighted to welcome Ji to the club having been a huge fan of the playmaker's talent prior to seeing her in an INAC Kobe shirt during the IWCC.
'She has got everything and is a great fit for our team and I've no doubt she will prove a big success in a Chelsea shirt,' added the Chelsea Ladies manager.