The aspiring fashion designer, who had 1.3 million followers on the platform, died at her flat in west London.
He tweeted: "Such incredibly sad news today".
Louis Tomlinson's younger sister had tragically passed away.
Police officers are investigating the teenager's death, which they are now treating as "unexplained".
The heartbreaking news comes just over two years after Félicité and Louis' mother, Johannah Deakin, lost her long battle with leukaemia.
U.S. singer Charlie Puth also tweeted, saying: "Prayers to Louis Tomlinson".
Scotland Yard said police were called by London Ambulance Service at 12:52 on Wednesday to a residential address following reports of a woman having a cardiac arrest.
Louis has not spoken publicly about this devastating news at this time, but he's reportedly "with family right now and he's devastated". "A female believed to be aged 18 was pronounced dead at the scene", the statement said.
Her last social media post came on Sunday (March 10).
Louis has cancelled his appearance on tonight's Comic Relief following the tragedy.
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