Aspiring fashion designer Felicite Tomlinson died at her flat in Earl's Court, west London, on Wednesday.
He tweeted: "Such incredibly sad news today".
The 18-year-old's death is now being treated as "unexplained".
Louis has not spoken publicly about this devastating news at this time.
The heartbreaking news comes just over two years after Félicité and Louis' mother, Johannah Deakin, lost her long battle with leukaemia.
X Factor winners Rak-Su also offered their prayers for the family.
Despite attempts to resuscitate Tomlinson, she was pronounced dead on the scene, TMZ said.
Family pictures show the singer with his fours sisters who have been supporting each other through hard times following the death of their mother. "A post-mortem examination will take place in due course".