Mark David Chapman -- who shot the Beatles member in front of Lennon's Manhattan apartment in 1980 -- has been serving 20 years to life after pleading guilty to 2nd degree murder.

Now 63 years old, Chapman will go before a parole board in New York sometime this week in a closed hearing ... and a decision will be announced in the coming weeks.

MDC has been denied parole 9 times since December of 2000.

Chapman shot Lennon 4 times in front of his wife, Yoko Ono in December, 1980 at the famed Dakota apartment building near New York City's Central Park ... and later admitted to planning the killing because he wanted to be famous.

He's been in prison since 1981.