The Downton Abbey creator has penned a new four-part mini-series about the 1912 disaster to be screened on ITV1.
But Fellowes told the Radio Times that the 1997 film, starring Kate Winslet and Leonardo DiCaprio, had unfairly portrayed William Murdoch, who was first officer on RMS Titanic.
The master mariner, who went down with the ship when she sank after hitting an iceberg, was credited with launching the lifeboats that saved 75 per cent of the survivors.
But in Cameron's Oscar-winning blockbuster, Murdoch, from Dalbeattie in Scotland, was portrayed as a coward who shot passengers before taking his own life.
Fellowes told the magazine: "That was very unfair how Murdoch was depicted. He wasn't cowardly.
"He fired the pistol to just stop a potential riot. It was suddenly getting out of hand, and he fired it in the air. That's not being cowardly."
Fellowes added: "I don't think you can just say, 'Well, we'll make this guy a villain - he'll do."
The Gosford Park writer told the magazine: "I think with real people you have a kind of imperative to be true to who they were.
"I don't think you can take someone who was moral and decent and make them do something immoral and indecent. I would feel uncomfortable doing that. So we do have Murdoch, and we have him firing a pistol...[But] there is a little bit of setting the record straight."
And just in case you forgot, this is what James Cameron's Titanic looked like - back in 3D soon!