Mediatakeout.com reports that a former girlfriend of Bennett's submitted the photos of the tight end, who was taking pictures of himself in a bathroom mirror.
"These pictures were taken four years ago and placed on the Internet recently without my knowledge or consent," Bennett said in a statement released by the Cowboys on Monday. "I understand that they are totally inappopriate. And for that I am sorry. I regret the embarrassment that it has caused the organization."
Bennett, who is on the non-football injury list with a sprained ankle, said he was aware of the pictures but declined further comments. Kennard McGuire, Bennett's agent, also declined comment.
Stephen Jones, team executive vice president, said he knows about the pictures but hasn't seen them and declined comment about them specifically.
"We discussed this matter today with Martellus and will address it further on an internal basis," Cowboys vice president of communications Rich Dalrymple said.
These photos of Bennett are just the latest jolt involving the third-year tight end, who has just 35 career catches.
Bennett has made more news for his off-the-field actions during his career. He was fined last year by the Cowboys for making a racially insensitive remark and for using a slur offending gays on a rap video posted on YouTube video. He used the same website to make another video depicting what an African-American Olympic games would look like that fed into stereotypes. He also made a Osama bin Laden video this summer.
"We fine players. We've done quite a few things in that area," Cowboys coach Wade Phillips said after Monday's practice about how the team has handled similar situations. "We talk about social media, and we have rules on all those things."
Bennett said last week that he was retiring from doing MartyBTV videos on YouTube and that he wanted to become more serious about football.
While working out on his own two weeks ago, Bennett sprained his ankle and has missed the first three training camp practices. He's expected to return soon.
"I've always had a business-like approach when it comes to football," Bennett said over the weekend. "I'm just chillin', just working on my game. I'm still the same person."
Bennett is not the first athlete to have this happen. In February, San Antonio Spurs guard George Hill apologized for having nude pictures of him appear on the internet, and Portland Trail Blazers center Greg Oden apologized for the same thing in January.