Officers were called to Pekar's suburban home by his wife about 1 a.m., Cleveland Heights police Capt. Michael Cannon said. His body was found between a bed and dresser.
Pekar had been suffering from prostate cancer, asthma, high blood pressure and depression, according to Cannon. Pekar had gone to bed about 4:30 p.m. Sunday in good spirits, his wife told police.
An autopsy was planned, said Powell Caesar, a spokesman for the Cuyahoga County coroner's office in Cleveland. He had no information on the cause of death.
Pekar's "American Splendor" comics, which he began publishing in 1976, chronicle his grousing about work, money and the monotony of life.
His quirky commentary developed a cult following and his insights and humor were often a bit on the dark side.
In 2003, the New York Film Critics Circle honored "American Splendor" as best first film for the directing-writing team of Shari Springer Berman and Robert Pulcini. Part feature and part documentary, with animated elements added, the film starred Giamatti as the disgruntled Pekar.
Pekar told The Associated Press in a 1997 interview that he was determined to keep writing his "American Splendor" series.
"There's no end in sight for me. I want to continue to do it," Pekar said. "It's a continuing autobiography, a life's work."