Mary O'Connor -- Hugh Hefner's loveable secretary/den-mother who was featured on "Girls Next Door" -- passed away this weekend ... after working at the mansion for more than 40 years.
Hefner tweeted the bad news Sunday ... saying, "Mary passed away today. We loved her more than words can say."
Sources at Playboy tell TMZ ... Mary -- who was reportedly in her 80s -- had been dealing with some health issues recently ... but Hef is still taking the death "extremely hard." We're told Mary was "the rock" of the Playboy mansion and the mood at the house is "incredibly sad."
Hugh's ex-GF, Kendra Wilkinson, also tweeted about Mary's death ... saying, "RIP Mary O'Connor. She was an amazing person who helped me through so much. But I know she's happy with her boo Captain Bob. :)"