The singer took to her Instagram Story account on Tuesday, June 18 to honour her late pal without actually naming him.
"Miss you," she wrote. "Hope you're okay up there. Long live you."
Eilish was targeted by haters for offering a heartfelt tribute to her friend after his death last year (2018), stating she didn't feel it was right to criticise someone "for loving someone that passed."
"I want to be able to mourn," she said. "I don't want to be shamed for it."
Meanwhile, XXXTENTACION fans flocked to the rap star's gravesite at the Gardens of Boca Raton Memorial Park in Florida early on Tuesday to mark the sad death anniversary.
The rapper, real name Jahseh Dwayne Ricardo Onfroy, was shot and killed outside a motorcycle dealership last summer.