Cliff Robertson, the square-jawed big-screen actor who nabbed an Oscar for “Charly” and played JFK in “PT-109,” died yesterday. He was 88.

His secretary, Evelyn Christel, said he died in Stony Brook University Medical Center on Long Island of natural causes a day after his birthday.

Robertson never rose to leading-man status but worked throughout his 50-year career in films and TV, including shows such as “Falcon Crest” and “Batman.” A younger generation knew him as Uncle Ben in the “Spider-Man” movies.

His signature performance came in 1968 with “Charly,” in which he played a mentally disabled janitor transformed into a genius.

He was also involved in real-life drama, taking the role of whistleblower in a 1977 check-forging scandal.

“My father was a loving father, devoted friend, dedicated professional and honorable man,” his daughter Stephanie Saunders said.

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