The heist was carried out by a gang of men disguised as police officers, officials said.
Kardashian West was left badly shaken, a spokeswoman for the star said, but was not physically harmed.
Here's what we know about the incident:
Where did it happen?
The heist took place in a rented apartment at a luxury private mansion in Paris' 8th arrondissement early Monday morning local time.
According to a French blog, the mansion has nine apartments that can be rented out for weeks at a time. The residence is hidden from the street by large wooden doors set in a stone wall, which is very common for private homes on the French capital's narrow streets.
The private mansion has proved popular with stars in the past as it is easy to avoid the paparazzi. Kardashian West's husband, rapper Kanye West, has stayed there previously, as have other celebrities.
How did the robbers get in and what was stolen?
The French Interior Ministry said the culprits were disguised as police officers. Five men threatened the concierge at the mansion with a weapon, handcuffed him and forced him to open the private apartment.
Two of the men were able to gain entry to Kardashian West's room, according to the Paris prosecutor's office, and a gun was held to her head.
A source familiar with the situation told CNN that Kardashian West pleaded for her life as she was threatened with the gun, and was bound and forced into a bathtub. Tape was placed over mouth.
Kardashian's two children, North West and Saint West, were in New York at the time, the source said.
The men took two cell phones and jewelry worth millions of dollars, according to the Interior Ministry. But only one of the phones belonged to Kardashian West, the source told CNN.
The prosecutor's office said a ring worth an estimated 4 million euros ($4.49 million) and a jewelry box with contents worth about 5 million euros ($5.6 million) were stolen.
It has not been disclosed whether this was her own jewelry or items she had borrowed for Paris Fashion Week, which she was in the city to attend.
Kardashian West owns several highly valuable items.
In addition to her original engagement ring from Kanye West in 2013, it was reported that he gave her a second ring last month with a huge stone, which she has been flaunting on social media.
Where was her security? And how was it planned?
Kardashian West has a regular bodyguard, Pascal Duvier, but CNN affiliate BFM TV has reported that she did not have a bodyguard with her at the time of the robbery.
Authorities in Paris have not commented on her security arrangements or said if anyone has been arrested in connection with the heist.
CNN's Jim Bittermann in Paris described the robbery as the work of "clearly an organized gang that has a lot of advance information."
How is Kim doing and where is she now?
Kardashian West left France Monday after speaking with police, the Interior Ministry told CNN. She was seen with a long black scarf over her head as she boarded a private jet, accompanied by bodyguard Duvier.
A spokeswoman for Kardashian West said the celebrity was "badly shaken but physically unharmed" following the incident.
What have the Kardashians said about it -- and what are people saying on social media?
Kardashian West -- a prolific user of social media -- has not updated her Twitter account since 11:30 p.m. Paris time on Sunday, when she plugged a Los Angeles promotional event.
This would appear to be before the robbery, which Bittermann reports happened around 2:30 a.m. Monday (8:30 p.m. ET Sunday evening).
None of her family -- also very prolific users of social media -- have made any comment so far.
Not everyone has expressed sympathy for Kardashian West.
But British comedian James Corden stuck up for her, taking to Twitter to urge people to "be nice or shut up."
Paris Mayor Anne Hidalgo condemned the robbery via Twitter.
"I express my support and I want to tell her that she will always be welcome in Paris. I have every confidence in the police force, as part of the ongoing investigation, to quickly identify and apprehend the perpetrators of this attack. This rare act of violence occurred in a private area, which does not call into question the work of the police and the security of public areas in Paris," Hidalgo said.