"It's really true that we never know what the day will bring us," the 28-year-old actress wrote in a touching tribute to her late pet. "My baby Frenchie Beau has gone to doggy heaven this afternoon."
Duff wrote that she would be posting tons of photos of Beau in the coming weeks, but with the loss so fresh in her heart she couldn't "bare to look at a photo of his little face that melted me every time."
"I can't fathom never holding his heavy little head in my hands with his two big eyes looking at me again," Duff continued in her emotional memorial. "Remember every day to be grateful for what we have that makes our hearts happy and full."
"My naughty boy I will miss you so," Duff concluded. "Heart. Broken."
Duff has been an avid animal lover for years. In January, Duff used her star power to help her friend, Julia, find her missing Maltese puppy after it was stolen in Beverly Hills.
This past September, Dwayne "The Rock" Johnson also suffered the loss of his French bulldog, Brutus, who died after eating poisonous mushrooms while playing outdoors. Watch the video below for more on the Furious 7 star's heartbreaking tribute to his late puppy.