The 22 year-old becomes Saints’ first recruit ahead of the 2015/16 campaign, joining from La Liga side Málaga.
Juanmi – whose full name is Juan Miguel Jiménez López – has penned a four-year contract and will link up with his new teammates when they begin pre-season training on Monday 29th June.
Executive Director of Football Les Reed commented: “Juanmi is a young, exciting and attacking player who fits into the club’s strategic plan perfectly.
“He has been tracked closely by our recruitment department for some time, and we were keen to act quickly when presented with the opportunity to bring this young player with international quality to Southampton.
“To have attracted a player of his potential at this early stage of the summer is testament to our determination to build upon the achievements of the 2014/15 season and to continue to implement our long-term plan for the future.
“We will fully embrace the challenge of the Europa League in the coming campaign, and want to develop further in Europe over the seasons to follow. Juanmi’s arrival is an important step in this process.”
Juanmi added: “It’s a pleasure to be here and I am looking forward to being a part of this club.
“I am anxious and I want to start as soon as possible. This is a big club and it has shown a lot of trust in me.
“The training ground is magnificent and likewise the stadium. It really, really is a big club.
“Since I was very young, it’s always been a dream for me to play in the Premier League. The style of football played here is similar to the way I play, so it’s a dream come true.
“Now I just want to start playing and to start scoring goals.”