Ruya Kadir died in hospital on Monday from injuries she suffered in the assault at an apartment complex in Boise, Idaho, on Saturday.
The girl had been celebrating her third birthday when a man entered the party and attacked nine people with a knife.
She was flown to hospital in Utah for treatment, but died there.
News of her death was revealed as the suspect, Timothy Earl Kinner, appeared in court to be charged with first degree murder, eight counts of aggravated battery and a use of deadly force enhancement.
Kinner, 30, is American, and the victims are members of refugee families from Syria, Iraq and Ethiopia – but Boise Police Chief William Bones said the evidence does not suggest the attack was a hate crime.
Chief Bones said Kinner, described in court as homeless, was staying with someone at the Wylie Street Station Apartments last week, but was asked to leave on Friday over bad behaviour and returned to take revenge for being kicked out.
Officers were were called at 9pm on Saturday and found stabbing victims inside and outside the apartments.
Kinner was arrested and taken to jail early on Sunday morning on suspicion of nine counts of felony aggravated battery and six counts of felony injury to a child.
In court he was said to live in Los Angeles and had no real ties to Boise.
He also has an extensive criminal record in Tennessee, according to court records, and has previous drug, weapons and assault convictions.
Ada County deputy prosecutor Dan Dinger said Kinner, who is being held without bail, has shown aggressive behaviour in jail, threatening staff and endangering himself and others.
Kinner asked to represent himself in court, but was assigned a public defender by Judge Russell Comstock.
“Do you understand what you’ve been charged with now, sir?” Mr Comstock asked.
“No, no sir,” Kinner replied.
“What don’t you understand?” the judge asked.
“I don’t understand none of this, sir,” Kinner said.
A vigil for the victims was held on Monday evening at Boise City Hall.