Shah Abbas Khan, father-of-two from south London, died on December 16, 2013 while in custody in Damascus.
The ortheopaedic surgeon had travelled from Turkey to Aleppo to treat injured civilians. He was arrested in November 2012 whilst volunteering in a hospital. His family campaigned for his release for months before he was found dead.
They claim he was murdered but the Syrian government maintained he was found hanging in a jail cell.
Under English law, inquests are held to examine violent, unnatural or unexplained deaths.
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The jury of seven men and four women concluded that the medical cause of Khan's death was "unascertained", but found his killing unlawful.
"Dr Khan was deliberately and intentionally killed without any legal justification," said the jury forewoman.
"As a family, we have always maintained that our brother was an innocent man who travelled to Syria for no other reason than helping injured civilians in the conflict," said Afroze, Khan's brother.
Khan's mother, Fatima, said she was grateful to the jury, but believed she had been let down by governments, including the UK.
"There was no justice in Syria like we have British justice here - no court, no justice - otherwise my son would have been released. I couldn't save my son. I trusted judges, lawyers and ministers but everyone lied to me. They stabbed me in my back," she said after the inquest.
"It is clear that he wanted to use his medical skills to help others, and that included helping others in conflict-torn Syria," said Judge Peter Thornton, the chief coroner.
The jury had been asked to consider whether Dr Khan took his own life, or if he was "forced in some way by his captors to take his own life against his will", or if he was "unlawfully killed" by his captors.
The family's counsel, Michael Mansfield QC, said it was an important case which laid down a marker and should now go forward to the International Criminal Court.