John Letts and Sally Lane denied funding terrorism but were convicted at the Old Bailey, as their son remains in Kurdish custody.
They were handed a 15-month prison term suspended for a year due to the “special circumstances of the case”, meaning they will not be jailed unless they commit further offences in that time.
Judge Nicholas Hilliard QC said: “It was one thing for parents to be optimistic about their children and I do acknowledge he is your son who you love very much. But in this context you did lose sight of realities.
“The warning signs were there for you to see.”
Speaking outside court, Letts and Lane said: “We have been convicted for doing what any parent would do if they thought that their child’s life was in danger.”
The jury found the pair guilty of sending Jack Letts £223 in September 2015, but acquitted them over another transfer in December the same year.
Lane and Letts tried to block the prosecution – taking their case to the Supreme Court – despite it being among numerous similar prosecutions of family members who sent money to relatives in Syria.
In February, a mentally ill man was jailed for two-and-a-half years for transferring £2,500 to his sister after she manipulated him into believing she was fleeing Isis in fear of her husband.
The court heard that Letts, an organic farmer, and former Oxfam fundraising officer Lane had ignored warnings that their son had joined Isis.
They bought him a plane ticket for a “grand adventure” in the Middle East after he converted to Islam as a teenager, despite being told he had been radicalised.
The jury was told that a man who knew Jack from their local mosque in Oxford urged Letts to confiscate his son’s passport after learning of his plans to leave the UK.
But on 24 May 2014, Ms Lane bought a ticket from London Heathrow to the Jordanian capital of Amman, and Jack – who was then 18 – flew out two days later.
He spent time in Jordan, Kuwait and Iraq, where he married the daughter of a tribal elder, before travelling into Isis territory in Syria.
Jack posted a photo of himself online in combat fatigues near Isis’s de facto capital of Raqqa, and openly wrote about his desire to carry out a terror attack on Facebook.
Commenting on a photo showing a former school friend with fellow British Army soldiers, he wrote: “I would love to perform a martyrdom operation in this scene ... I would happily kill each and every one of Linus Unit personally. This message for you, mum and [younger brother] Tyler, I honestly want to cut Linus head off.
“I hope he finds himself lost in Beji or Fallujah [cities in Iraq] one day and sees me whilst I’m armed and I will put six bullets in his head.”
In March 2015, police visited the family home, seized their computers and devices and the couple were formally warned that they could be prosecuted for sending their son property or money, jurors were told.
When he found out about the raid, Jack responded: “Please convey to the British police that I’m not planning on coming back to their broken country ... convey to them from me ‘die in your rage soon you’ll be the ones being raided!’.”
The defendants also consulted an academic expert who told them it was “highly improbable” that Jack was not fighting for Isis.
In spite of the mounting evidence, Lane sent £223 after Jack Letts gave her his word the money would have “nothing to do with jihad”.
Police followed up with a second warning, telling Lane that “sending money to Jack is the same as sending money to Isis”.
But in December 2015, Jack began indicating he would like to leave Syria and told of a “big misguidance in the state”.
Letts told a family liaison officer that his son was “desperate to get out” and in “danger”, and was advised he could send him money to leave.
The advice was quickly corrected and the defendants were issued with a written notice stating: “The police do not endorse or authorise the sending of any monies to Jack Letts.”
Lane told her son: “We know you are in danger so we feel we have no choice but to help you and send it.”
She went on to attempt two money transfers which were blocked, and the defendants were arrested.
Letts declined to give evidence, but his barrister Henry Blaxland QC told jurors the prosecution was “inhumane to the point of being cruel”.
Prosecutor Alison Morgan QC told the jury the parents ignored “clear warning signs about what Jack was doing” and advice from police, academics, charity workers and well-wishers that it sending money could result in prosecution.
The defendants, from Oxford, denied three charges of funding terrorism.
They claimed their son, who suffers from obsessive compulsive disorder, was trapped in Raqqa and by December 2015 when they tried to send more funds, they were acting under “duress” fearing he was in mortal danger.
Lane wept in court as she described her “horror” when her son told her he was in Syria, while Mr Letts begged his son to come home and told him: “You don’t have to die to help your fellow Muslims.”
The father accused his son of becoming a “pawn ... helping spread hatred, pain, anger, suffering and violence”, jurors heard.
Both parents had exchanged emotional messages with their son, seeing Lane admit: “Clearly I indulged you ... I was a terrible parent that gave you too much power as a child.”
A jury deliberated for nearly 20 hours before they reached their verdicts on Friday, and were discharged after they were unable to decide on a third charge relating to an attempt to send money in January 2016.
Ms Morgan said the Crown would not seek a retrial and asked for the charge to lie on file.
Detective Chief Superintendent Kath Barnes, head of Counter Terrorism Policing South East, said investigators understood the “anguish” Letts and Lane must have suffered.
“You absolutely understand the bond between parent and child but you can’t break the law and in this case there were a lot of warnings, a lot of signs and a lot of explicit concerns given to John and Sally,” she told The Independent. “It was just that they almost didn’t want to see it.”
DCS Barnes said the cause showed that “it really doesn’t matter who you are or what your reasons are, it’s not OK to break the law”.
“We don’t want any money, however large or small, to fall into the hands of terrorists because we know what they use that money for,” she added. “They use it to go off and commit further atrocities, kill, maim, terrorise people. We saw the attacks of 2017 and the devastation caused on the streets of the UK, so the amount of money to some extent is completely irrelevant.
“Our job is to continue to work to mitigate all of those threats against us in the UK and our interests overseas.”
Jack, now 23, remains in Kurdish custody accused of being an Isis member.
Additional reporting by PA