The 22 year old, who was officially unveiled by Saints boss Mauricio Pochettino yesterday, has signed a four year deal worth in the region of £30,000 per week.
The transfer fee matches that paid by the South Coast club for Uruguay international Garston Ramirez last summer and eclipses the previous record amount of £9.5million received by the Scottish Premier League champions for Aiden McGeady, who moved to Spartak Moscow in 2011.
The midfielder, who cost Neil Lennon just £900,000 two years ago, shot to prominence following last season’s Champions League run.
His goal in the historic 2-1 win over Barcelona last November saw him draw interest from a number of European heavyweights including Manchester United and Liverpool.
He becomes Southampton’s second signing of the summer, following last month’s arrival of Croatian defender Dejan Lovren.