Pompey have signed Nugent and John Utaka for undisclosed fees but the pair are estimated to have cost £13million to bolster Harry Redknapp's attack.
Sunderland withdrew their bid for Nugent, who scored on his full England debut and was a regular for the Under-21s, and Redknapp persuaded the 22-year-old to join a squad looking to improve on last season's top-half Premier League finish.
Nugent said: "The Premier League is where you need to be to further your career.
"Portsmouth have a great squad and hopefully I can help them get into Europe.
"You can see the squad - great names and great signings, and I'm looking forward to joining up with them.
"There's a mixture of experience and younger players here, which you need in the squad.
"I can achieve a lot here. I've got to break into the senior [England] squad as I can't play for the Under-21s anymore."
Nugent was also linked with Everton but a move never materialised.
"They're my boyhood club but I'm not going to wait around for ever," he added.
Utaka, 25, was subject to late interest from other English clubs and his club, Rennes, did not want to let him go.
"I spoke with Kanu and he told me this is a good place," said the Nigeria international who, like Nugent, has signed a four-year contract.
Kanu's future at Fratton Park appears less certain. The veteran striker is out of contract and has been offered another year with an added season if he features in 20 games.
Manager Redknapp said: "He's still got a part to play, he can come on and turn a game.
"He's worried about 20 games. If you don't play 20 games, you're not worth another year really. He'll play 20 I'm sure."
Regardless of whether Kanu commits to the club, Redknapp wants to sign another attacking player before the start of the campaign.
"I've got a couple of options," said Redknapp. "Another forward, maybe a wide man."