Legendary singer Neil Sedaka's cause of death was "atherosclerotic cardiovascular disease," according to his death certificate.

According to the American Heart Association, the condition is caused when plaque builds up in artery walls ... and it's linked to heart attacks and strokes.

The death certificate -- first obtained by the California Post -- also cited kidney failure as a factor in Sedaka's passing last month.

TMZ broke the story ... the crooner was rushed to the hospital because he wasn't feeling well. He died the following day, on February 27, in L.A.

At the time of his death, Neil's family told us they were "devastated" by his "sudden passing."

In the '60s, the songbird topped the charts with songs like "Oh! Carol," "Calendar Girl," and "Breaking Up Is Hard to Do."

In 1983, Sedaka was inducted into the Songwriters Hall of Fame and amassed five Grammy noms over the course of his career.

He left a literal mark on Hollywood ... receiving a star on the Walk of Fame in 1978.

Neil was 86.

RIP