The Easy A actress first revealed that her 4-month-old teacup Pomeranian, Little Angel, had gone missing via her Twitter page on Tuesday.
"Everyone: I have the worst news to tell you.... Little Angel got out of my house somehow and is LOST (sic)," she wrote. "If anyone finds her I BEG you to please contact me thru twitter, I will give a reward. She means the WORLD to me & I can't find her anywhere."
She also posted several photos of the pet as she urged fans to help ensure the animal's safe recovery.
However, she returned to the social networking site on Wednesday to announce that the dog had been found dead, though she declined to offer details on what had happened.
"Sad day. Little Angel is in heaven now. RIP I love you," she posted.
Bynes previously announced her retirement from showbiz, only to later reverse her decision.