The film legend made the appeal in one of several letters of support filed Friday in federal court in Manhattan on behalf of Cameron Douglas.
Other letters were penned by his celebrity stepmother, Catherine Zeta-Jones, NBA executive Pat Riley and longtime friends who described their despair over watching Cameron waste his talent as a budding actor. He had roles in movies including 2003's "It Runs in the Family," starring his father and grandfather.
Cameron Douglas, 31, is "a considerate, worthy human being," Zeta-Jones wrote.
The son of Academy Award winner Michael Douglas pleaded guilty earlier this year to charges he dealt methamphetamine out of a trendy Manhattan hotel.
The charges carry a minimum term of 10 years behind bars. But lawyers for the admitted heroin addict claimed in court papers their remorseful client was committed to sobriety, and argued he only deserves a maximum 3 1/2 years at sentencing next week.
In his letter, Kirk Douglas recalled how he recently traveled from Los Angeles to New York - "At 93, that's a long trip" - to visit his jailed grandson.
"He didn't express any self-pity, nor did he ask for any," he wrote. "The only sorrow he expressed was for the trouble he had caused others. ... I was shocked when he got in such a mess."
The famous grandfather concluded: "I'm convinced Cameron could be a fine actor. ... I hope I can see that happen before I die. I love Cameron."
Riley, president of the Miami Heat, said he got to know Douglas while coaching the Los Angeles Lakers.
"Cameron is not a criminal," Riley wrote. "I ask for mercy, judge."
Douglas was arrested July 28, 2009, at the Hotel Gansevoort as part of a Drug Enforcement Administration probe.
Investigators say Douglas was paid tens of thousands of dollars for smuggling meth - what he referred to in intercepted phone calls as "pastry" or "bath salts." Cash and drugs were routinely exchanged from coast to coast through shippers like FedEx, a criminal complaint said.
In the papers filed Tuesday, defense lawyers said Douglas had come to New York to look for an apartment when he had a chance encounter outside the hotel with a fellow guest and drug dealer who invited him to dinner.
Afterward, "the New York dealer asked Cameron to discuss a business proposition and invited him to his hotel room, where the New York dealer asked him if he could send him a pound of meth," the papers said. "Still impaired by his addiction, Cameron agreed and later flew to California to arrange the requested transaction."