The letter, published in two Jordanian newspapers on Sunday, was delivered through the International Committee of the Red Cross to an old friend of Saddam's now living in Jordan.
Those who made the letter public said the man refused to be identified.
It was believed to have been the first letter since Saddam was captured in December 2003 sent to someone other than a family member.
"My soul and my existence is to be sacrificed for our precious Palestine and our beloved, patient and suffering Iraq," the writer of the letter said.
Tayseer Homsi, secretary-general of the Jordanian Arab Baath Socialist Party, said the missive had been delivered through the ICRC to an "independent Jordanian political figure who wished to remain anonymous".
Rana Sidani, a spokeswoman for the ICRC's Iraq delegation in Amman, said: "The ICRC has confirmed the authenticity of the message published in the Jordanian media."
"The ICRC collected the message," she said. "It was censored by the detaining authorities before being handed over to the ICRC for distribution."
"My soul and my existence is to be sacrificed for our precious Palestine and our beloved, patient and suffering Iraq"
A friend of Saddam's family, speaking on condition of anonymity to avoid straining relations with them, said the "handwriting is 100% Saddam's".
Saddam's two daughters, Raghad and Rana, live in Amman after having fled the US invasion two years ago.
The ousted leader was expected to stand trial in Iraq this fall on charges that could bring the death penalty. His letter appeared to include his musings on that possible fate.
"Life is meaningless without the considerations of faith, love and inherited history in our nation," the letter said.
"It is not much for a man to support his nation with his soul and all he commands because it deserves it since it has given us life in the name of God and allowed us to inherit the best," he wrote in a what appeared to be a clear call to Arabs to follow in his footsteps.
"My brother, love your people, love Palestine, love your nation, long live Palestine," the document concluded.
The Jordanian Baath party, which publicised the letter and espouses ideology similar to Saddam's now-defunct Baath Party, has no formal links to Iraq.
Homsi, the party chief, said the letter's recipient gave his party a copy of the letter two days ago.
"The Jordanian man wished to remain anonymous, he's an old friend of Saddam, he's not a member of our party nor is he a party functionary," Homsi said, declining to identify the man.
The letter became public as Iraq geared up for a series of trials, the first beginning this August, which involves Saddam's alleged role in the 1982 massacre of an estimated 150 Shia in Dujail, north of Baghdad and in retaliation for an assassination attempt on the former leader.
Saddam is a Sunni and his minority sect in Iraq ruled over majority Shia, Kurds and other ethnic groups until he was ousted in April 2003 after the US-led invasion.
Others indicted in the Dujail massacre are Barazan Ibrahim, intelligence chief at the time and Saddam's half brother; former vice-president Taha Yassin Ramadan; and Awad Hamed al-Bandar, at the time a Baath Party official in Dujail.
The letter exhorts the reader to
'love Palestine, love your nation'
The assassination attempt was organised by the Dawa Party, whose members include current Prime Minister Ibrahim al-Jaafari.
Also on Sunday, Iraq criticised Jordan for allegedly allowing Saddam's family to fund an Iraqi network seeking to destabilise the country.
The rebuke appeared designed to blunt bad publicity for Iraq after Jordanian police detained an undetermined number of Iraqis and other foreign Arab suspects in the Friday rocket attack that barely missed a US warship in Jordan's Red Sea resort of Aqaba.
Ad-Dustour and Al-Arab Al-Yawm, Jordan's second and third largest dailies, said the letter was given to them by Homsi's party at a press conference on Saturday.