The Advertising Standards Authority has received 33 complaints that the campaign, with the tagline "I am what I am", is sending out the wrong message to teenagers that gun crime is acceptable.
An ASA spokeswoman confirmed the complaints are being investigated, saying: "They are concerned that it glorifies the US-style gang culture and feel that is disgraceful."
The campaign, which includes a TV ad by New York agency mcgarrybowen involving the rapper counting to nine in reference to the number of times he has been shot, has already come in for criticism from the anti-gun lobby.
On its UK launch last month, Bill Brown, chairman of the Disarm Trust charity, described the ads as "irresponsible and despicable".
A Reebok spokesperson said on the launch: "Reebok does not condone every action, choice or view expressed by the athletes and entertainers who wear our products."
She added that the ads featuring 50 Cent would only be shown during TV shows and in magazines that are "adult-oriented".
The £12m integrated campaign features a raft of celebrities, including Jay-Z and Dame Kelly Holmes.
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