Ramirez, 21, has been linked with the likes of Liverpool and Tottenham this summer but has decided to move to the newly promoted side.
The Uruguay international says he is excited by the prospect of moving to the Premier League side in a transfer that, if completed, would be a huge coup for Southampton.
“They believe in me in every way,” the player told the Corriere dello Sport. “And above all I have said that they are going to grow.
“Southampton is a newly promoted team in the English top flight, but they have the means to stand tall in a few years. And with me they are going to build.
“It’s all very stimulating – even if it is a little strange.”
The forward scored eight goals and claimed four assists in Serie A last season.