Pelle has notched 55 times in two years for Dutch side Feyenoord where he worked with new Saints boss Ronald Koeman.
Despite the sales of Rickie Lambert, Adam Lallana and Luke Shaw, coupled with the possible departures of Morgan Schneiderlin and Dejan Lovren, Pelle believes Southampton are one of the fastest improving clubs in Europe.
“I’m really happy that it’s finally signed,” said Pellè moments after putting pen to paper.
“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.
“The talks between us were only short in terms of how long it took to convince each other. I was pretty sure that, for me, this would be a great step.
“[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.”