The 24-year-old SAG team rider crashed at Turn 12 at the challenging Montmelo track near Barcelona with 25 minutes left in Moto2 Free Practice 2.
"Two medical cars and then two ambulances were immediately on scene to treat the 24-year-old and the medical helicopter was also deployed to assist in his treatment," organisers said in a statement.
"Due the severity of his condition, the decision was taken to transfer Salom by road to the nearby Hospital General de Catalunya.
"On arrival, he underwent surgery, but despite the best efforts of the trauma team, he passed away at 1655 local time (1455 GMT)."
Salom made his world championship debut at Jerez in 2009 in the 125cc category, and finished on the podium 25 times -- with nine wins in the Moto3 category.
He was runner-up in the 2012 Moto3 world championship and third overall in 2013.
He also finished on the podium three times in 41 appearances in the Moto2 world championship, including a second place in the opening round of 2016 at Losail in Qatar.