But the jihadists still hold areas on the east bank in the north of oil-rich Deir Ezzor province, the Syrian Observatory for Human Rights said.
"Daesh no longer has a presence on the western side of the river," Observatory head Rami Abdel Rahman told AFP, using an Arabic acronym for IS.
"Its biggest presence is on the eastern side of the Euphrates, where it still holds around eight percent of Deir Ezzor province," he said.
East of the Euphrates, IS is under attack by both government troops and Kurdish-led militia.
IS also has a presence in eastern parts of the central provinces of Homs and Hama, and in the Yarmuk Palestinian camp near Damascus, while an allied group holds positions in the south.