Rapper Jay-Z thinks success has gone to Rihanna’s head – so he’s hired a bodyguard to watch over his chart-topping protégé.

The Grammy-winning hip-hop artist "invested his heart and soul into transforming Rihanna into a superstar," an insider told The ENQUIRER.

"Now he’s outraged because she’s out clubbing most every night. He thinks she’s losing her professional edge."

Music mogul Jay-Z plucked Rihanna from obscurity in2004, signing her to his former record label Def Jam.

She scored huge hits with "Umbrella" and "SOS," but in February, the 21-year-old Barbados-born singer made worldwide headlines after a brutal attack by her former boyfriend, Chris Brown.

Her behavior since then has Jay-Z, so concerned that he recently assigned one of his own bodyguards exclusively to Rihanna, said the source.

"Jay-Z’s take is that Rihanna’s been on a downward slide ever since the Chris Brown beating," the source said.

"He wants to keep an eye on Rihanna to make sure she doesn’t wind up in trouble. He’s unhappy that she appears to prefer partying to making music."

After the MTV Video Music Awards on Sept. 13, Rihanna partied up a storm at New York City hotspots 1OAK and Jay-Z’s 40/40 Club.

Days before, she was seen throwing back shots at M2 Ultralounge in the Big Apple.

Wonder if the party guard’s joining in the festivities?