"I can think of nothing more despicable. I regret having made these comments, and apologise to all."
Grisham, who has sold more than 275 million books during a 25-year career, made the comments while talking about a law school friend convicted of sex offences.
"His drinking was out of control and he went to a website," he said. "It was labelled '16-year-old wannabe hookers' or something like that.
"So he went there. Downloaded some stuff - it was 16-year-old girls who looked 30. He shouldn't have done it. It was stupid, but it wasn't 10-year-old boys. He didn't touch anything. And God, a week later there was a knock on the door: 'FBI!' And it was a sting set up by the Royal Canadian Mounted Police to catch people - sex offenders - and he went to prison for three years.
"We have prisons now filled with guys my age. Sixty-year-old white men in prison who've never harmed anybody, would never touch a child.
"But they got online one night and started surfing around, probably had too much to drink, and pushed the wrong buttons and got into child porn."
Grisham insisted that he had "no sympathy for real paedophiles" but added: "So many of these guys do not deserve harsh prison sentences."
His comments were widely reported in America and sparked angry criticism on social media.
"To John Grisham: Every child photographed is a victim and must live with an ever-present internet reminder," wrote one Twitter user.
Another wrote: "John Grisham can bite it. People who view child porn perpetuate a horrible cycle and your 'buddy' was a creep."
His comments quickly became one of the most-discussed topics on the social network.
America has in recent years increased sentences for those who view child pornography, largely in response to pressure from campaigners.
Since 2004, average sentences in the United States for those who possess - but do not produce - child pornography have nearly doubled, from 54 months in 2004 to 95 months in 2010, according to a 2012 report by the US Sentencing Commission.
However, the lengthy sentences have raised questions in American legal circles after it emerged that people who viewed child pornography were sometimes facing harsher sentences than those who physically abused children.