The swimming champ took to Instagram to share the sad news that Lester, who she adopted in 2006, had to be put to sleep due to his failing health.
"I adopted Lester from @lortsmith only a couple of weeks after hanging up the togs in 2006, he was 8 weeks old, had just had his leg amputated & was looking for a new home... it was love at first sight," she wrote.
"Lester & I were inseparable, & as a Lort Smith Animal Hospital Ambassador he went everywhere with me, charming everyone he met due to his spunk, smoochiness & intelligence."
Rooney explained that in the last six months Lester's had aged very quickly with his leg causing him a lot of pain, despite being on medication.
"He was deaf, mostly blind & becoming disorientated, but when he started throwing up & I took him to emergency, I still thought that he would just need some fluids & anti nausea medication & he would be fine, how wrong I was.
"After a drip, blood tests & an ultrasound it was discovered that Lester had a small liver, spots on his kidneys (would need a biopsy to discover exactly what these were) & most alarmingly a severely inflamed gall bladder that needed removing immediately... in most cases this surgery would be successful but considering Lester’s age".
The 35-year-old was forced to make the 'hardest decision of her life' and chose to put him to sleep.
Before he went, Rooney thanked him for his unconditional love and enriching her life, and told him he would never be forgotten.
Sending love to Giaan.