The Italian forward becomes new Saints boss Ronald Koeman's second transfer of the summer, following Dusan Tadic to St Mary's for a fee believed to be in the region of £8 million.
The 28-year-old has worked with Koeman before, moving from Parma to the Eredivise club in 2012 and links up with the Dutch legend for a second time.
Speaking about his move, Pelle told the club's official website: "I’m really happy that it’s finally signed.
"I had some teams interested in me, but this is going to be the best choice for me because the club wants to keep improving every year.
"[Ronald] Koeman and I worked together for two amazing years at Feyenoord, and I hope that it will be the same here.
"There are many good things about this club other than the trainer. The Premier League is amazing, and Southampton is showing year-by-year that it wants to improve.
"Last year, everyone in Europe was impressed by the performances of the team, and it’s clear that the club is growing. It’s great to be here.”
Southampton have been active in the transfer market so far this summer, selling Adam Lallana and Rickie Lambert to Liverpool and Luke Shaw to Manchester United after an impressive season under Mauricio Pochettino.
And with the club looking to rebuild, Koeman expressed his delight at capturing a player he rates highly.
He said: "I had Graziano in my team for my last two years at Feyenoord. He scored a lot of goals – more than 50 in two seasons – and they brought a lot of points to the club.
"Goals mean points, and we needed a good player in that position after Rickie Lambert left.
"In terms of his qualities, he’s a tall striker with a lot of movement and he’s fast. He’s a player who gives confidence to the team because he keeps the ball, and he’s very dangerous in front of goal.
"He’s almost 29 years old, so he brings a little bit of experience to a young team.
"Overall, we’re pleased to have brought in Graziano after signing Dusan Tadic earlier this week. This shows we are committed to strengthening the team and progressing after last season.
"We know that we need some more players because we haven’t occupied all of the positions. We need more players to bring the squad to where we need and expect."
Pelle registered 55 goals in 66 games for Feyenoord in his two-year spell at the De Kuip.