Kim Kardashian arrived in Paris, France today on a private plane ... but not for a jet-set luxury visit -- she's set to testify tomorrow in a long-delayed trial, after she was robbed at gunpoint in the biggest jewelry heist in France in decades.

Kim flew into Paris-Le Bourget Airport Monday afternoon ahead of Tuesday's hearing -- a group known as the "Grandpa Robbers" is charged with stealing more than $10 million in jewelry from her at a fancy hotel in the city in 2016.

Kim was visiting back then to attend Paris Fashion Week ... and at about 3 AM on October 3, two armed robbers overpowered a security guard outside her Hôtel de Pourtalès apartment. She was bound with zip-ties and duct tape, then deposited into a bathtub, as two masked men took her jewelry -- including a $4-million engagement ring given to her by then-husband Kanye West.

Nine men and one woman -- most are in their 60s and 70s now -- are standing trial for planning and executing the robbery plot nearly a decade ago, though the suspects were arrested mere months after the heist. Five are charged with armed robbery and kidnapping, the others are accused of being accessories. One suspect died recently, and still another is suffering from advanced dementia.