Chiuri had presented her last collection for Valentino only the night before, taking the bow at its Paris haute couture show with her longtime creative partner Pierpaolo Piccioli.
“I have shared with Pierpaolo a great part of my professional life,” she said in a statement.
“I am ready to embark on a new professional challenge.”
Dior refused to comment on whether the 52-year-old had been lined up to follow in the footsteps of the house’s legendary founder Christian Dior and such designers as Yves Saint Laurent, Gianfranco Ferre and John Galliano.
But sources told AFP that a announcement is likely to be made Friday.
Dior has been without a leader since the shock departure of the Belgian Raf Simons last October, which sparked soul-searching in the industry about the pressure creators were now under.
Valentino said Piccioli would now take sole charge of the label.
Chiuri and Piccioli arecredited with giving Valentino back its fizz, making it one of the most profitable designer brands in Europe.
Her fairytale floor-sweeping dresses and willowy frocks have won her an army of Hollywood fans including actresses Gwyneth Paltrow, Anne Hathaway and Kiera Knightley.
With her departure widely predicted, Vogue described Chiuri’s swansong Paris show for Valentino as “an unforgettable farewell”.
Its Shakespearean ruffs and and Renaissance-inspired dresses had the critic Suzy Menkes asking archly, “To stay or not to stay, that was the question.”
But a source had earlier told AFP that Chiuri already signed on the dotted line with Dior.
Chiuri, whose mother was a dressmaker but whose parents strongly disapproved of her going into fashion, took over at Valentino with Piccioli when its colourful founder Valentino Garavani retired in 2008.
Valentino, the “Sheikh of Chic”, had poached them from the rival Roman label Fendi nearly a decade earlier.
There the pair had pioneered its ground-breaking accessories range, with Chiuri credited with creating its distinctive studded handbags.
Although there has been much speculation about how their 25-year partnership works, Chiuri was always regarded as the senior partner.
Piccioli, who is two years her junior, has often praised her famous “intuition”.
“Her faults? Can I make a list,” he told the Italian daily La Repubblica.
“Sometimes she has a tendency to have her own way,” he joked.
Her love of exquisitely embroidered creations is legendary, with the silken applications on one of the gowns in Wednesday’s Paris show taking 480 hours to attach.
Some, however, have questioned her love of mediaeval motifs and called her look “anti-sexy”.
Other fashionistas claim this was deliberate decision to mark her out from the ultra sexual style of her Roman neighbour and competitor Donatella Versace.
In any case, unlike Versace, she claims not to have been a natural show-woman, recalling her first show in charge of Valentino as a “nightmare”.
“I was so shy and I found it hard to even talk. The night before I was at home trying to learn English from books” so that she could talk to the media.