Scott, 49, a former model turned fashion designer, was found dead in her luxury apartment in Manhattan on Monday.
Following an autopsy carried out on Tuesday, the New York coroner's office confirmed what had been widely reported since news of Scott's death broke.
"Decedent: L'Wren Scott. Cause of death: Hanging. Manner of death: Suicide," a coroner's office spokeswoman confirmed to AFP by email.
Jagger, 70, is reported to have traveled to New York following Scott's death, which forced the indefinite postponement of the Rolling Stones' tour of Australia and New Zealand.
The veteran rocker on Tuesday said he was "struggling to understand" why Scott, who he had been dating for well over a decade, would take her own life.
It has emerged that Scott's fashion design business was millions of dollars in debt at the time of her death.
A spokesman for Jagger has dismissed a report in the New York Post tabloid which said the rocker had recently broken up with Scott.
"This is a complete and utter fabrication and a result of ill-informed gossip at this tragic time for Mick. It is absurd and not true," Jagger spokesman Bernard Doherty said.