Segev, a 49-year-old physician, was arrested in Israel last April.
His lawyer said Segev and the prosecution agreed to a plea bargain. Segev, who belonged to the now-defunct, right-wing Tzomet party, initially denied all charges.
"He admitted to an attempt to smuggle ecstasy into Israel and holding a fake diplomatic passport at (Amsterdam's) Schiphol Airport," said the attorney, Zion Amir. "It is up to the judge to determine whether (his sentence) will be five years or less," he said.