Born in 2011 as the runt of the litter, Lil Bub found viral fame thanks to her distinctive appearance - her tongue often seen hanging out of her mouth.
Bridavsky broke the news to Lil Bub's 2.3 million followers, sharing a photo of his first photo with the famous cat alongside their last photo together.
He wrote: "On the morning of Sunday, December 1st 2019 we lost the purest, kindest and most magical living force on our planet.
"BUB was cheerful and full of love laying in our bed with us Saturday night, but unexpectedly passed away peacefully in her sleep."
Bridavsky explained that, as many followers knew, Lil Bub was 'battling a persistent and aggressive bone infection'. He said he felt she had made the decision to 'leave her failing body' so that he and his family wouldn't have to make the difficult decision to put her to sleep themselves.
Speaking of Lil Bub's impact, both on himself and the rest of the world, Bridavsky continued: "She taught me everything that I know about unconditional love, she brought my wife Stacy and I together, she's the reason we have our beautiful children Rosco and Lula, and she has been a constant source of warmth and love in our lives for the past 8 years. To say that our family is devastated would be an understatement.
"But most importantly, BUB has made a huge difference in the world of animal welfare, and in the lives of millions of people worldwide.
"She has literally saved thousands of lives (both pets and humans), she started the first national fund for special needs pets, she was the subject of groundbreaking genetic and biological research, she's helped raise over $700,000 for animals in need, and has spread a message of determination, positivity, and perseverance to people all over the world.
"And even though my heart is absolutely crushed by her graceful departure from planet Earth, I know that her spirit, magic, and overwhelming energy are still with us, reminding us every day to be better."
Bridavsky added: "Dearest BUB, I will never forget your generosity, your limitless supply of love, or your uncanny ability to bring so much magic and joy to the world. I am forever honored and humbled that you chose me as your caretaker."
RIP, Lil Bub.
Featured Image Credit: Instagram/@iamlilbub