A French police dog called Diesel was killed in Wednesday's raid on a flat in the Paris suburb of St Denis.
The operation was targeting the person who French police say was responsible for organising Friday's gun and bomb attacks in Paris, in which 129 people were killed.
"Diesel, a seven-year-old Belgian Shepherd, a female assault dog, was killed by terrorists during the operation," said the Twitter account run by French police.
Another message read: "Assault and explosives detection dogs are vital in the operations of RAID [a special French police unit]."
The hashtag #JeSuisChien (I am a dog) was also trending on French Twitter after the announcement, as was #JeSuisDiesel (I am Diesel).
Two suspected Islamist militants were killed during the raid.
Local residents were woken by automatic gunfire and explosions at around 4am.
One of them was a woman who detonated an explosives belt, which also killed the police dog.
Seven other people were arrested and five police officers suffered minor injuries.
It's still unclear if the man French police were hunting, Abdelhamid Abaaoud, was one of those killed.
Prosecutor Francois Molins said intelligence indicated he was in Paris.
The operation comes as part of a manhunt to find any links to Friday's attacks which left 129 dead and hundreds injured in a concert hall, bars and restaurants and outside France's national stadium.