Russian prosecutors claimed that Mr Hubbard had joined Ukrainian forces in Izyum as a mercenary.
He was accused of signing a contract worth around $1,000 a month, according to Russian state media.
His sister, Patricia Hubbard Fox, rejected the allegation as a lie.
Speaking ahead of his mission to Russia, US special envoy Steve Witkoff said he would push for the “release of wrongfully detained American citizen Hubbard, reflecting President Trump’s continued commitment to bringing Americans home.”
