The 25-year-old announced the sad news on her Twitter page today.
Bynes wrote: 'Sad day....Little Angel is in heaven now. RIP I love you.'
The incident occurred after Amanda appealed to fans for help in finding her lost dog after the pet went missing in Los Angeles.
She asked her Twitter followers to help her find the pup.
The actress wrote: 'Everyone: I have the worst news to tell you....little angel got out of my house somehow and is LOST.
'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 then posted a picture of the dog on the social networking website in a bid to try and find the pup.