Cardiff will allow long-serving Kevin McNaughton to leave this summer along with Danny Gabbidon and Nicky Maynard.
Highly-rated Atletico Madrid midfielder Saul Niguez has signed a new five-year contract.
The 32-year-old initially joined the Saints in 2010 on loan from Italian side Cesena, before that move was made permanent.
Do Prado has started just three league games in the last 12 months, and he has accepted that he will be departing St Mary's in the summer when his contract expires.
"I am going to miss Southampton if I have to go. I would be sad to go," said do Prado.