He was found unresponsive at his Manhattan apartment on 29 June and was pronounced dead at the scene, police said.
The son of a steel-mill worker, Ditko was born in Johnstown, Pennsylvania, in 1927. He served in the army in Europe after World War Two and began working in comics in the 1950s in New York.
Ditko, along with Stan Lee, introduced the world to Peter Parker and his alter-ego Spider-Man in 1962 in an issue of Amazing Fantasy.
A year later, Ditko introduced the world to surgeon-turned-metaphysical superhero Doctor Strange.
Ditko left Marvel in 1966 and returned the following decade.
One of his later creations was Squirrel Girl, a cult favourite among comics fans.
Ditko maintained a writing studio in Manhattan until his death, but he was incredibly reclusive, turning down nearly all offers to do interviews, meet fans or appear at movie premieres.
Dan Buckley, president of Marvel Entertainment, said: “Today, the Marvel family mourns the loss of Steve Ditko.
“Steve transformed the industry and the Marvel Universe, and his legacy will never be forgotten.”
“Thank you Steve Ditko, for making my childhood weirder,” Neil Gaiman, author of novels and comics, said in a series of tweets.
“He saw things his own way, and he gave us ways of seeing that were unique. Often copied. Never equalled. I know I’m a different person because he was in the world.”