Defence secretary Gavin Williamson confirmed the individual died after crossing into the strike area on a motorbike moments before the blast on 26 March.
It is the first time the Ministry of Defence (MoD) has confirmed a civilian casualty during Britain’s campaign against the terror group in Iraq and Syria.
The death was “deeply regrettable”, the minister said.
Three IS fighters, who were driving in the Syrian Euphrates valley at the time, were also killed by the precision Hellfire missile, a statement from the MoD at the time reported.
The attack was carried out by an RAF Reaper drone, which had tracked the trio.
It was one of more than 1,600 air strikes carried out against IS targets by the RAF in Iraq and Syria, Mr Williamson said.
His statement read: “We do everything we can to minimise the risk to civilian life from UK strikes through our rigorous targeting processes and the professionalism of UK Service personnel.
“It is therefore deeply regrettable that a UK air strike on 26 March 2018, targeting Daesh fighters in eastern Syria, resulted in an unintentional civilian fatality.
“During a strike to engage three Daesh fighters, a civilian motorbike crossed into the strike area at the last moment and it is assessed that one civilian was unintentionally killed.
“We reached this conclusion after undertaking routine and detailed post-strike analysis of all available evidence.”
An investigation will be carried out by the wider coalition, which will report “in due course”.
Britain’s direct military involvement against Islamic State, also known as Daesh, is limited to the skies, but troops on the ground have been involved in training more than 60,000 members of the Iraqi security forces.
Intelligence, surveillance and reconnaissance has also been provided to coalition partners, according to Mr Williamson.
“These strikes are undertaken in the collective self-defence of Iraq as part of the global Coalition to defeat Daesh, and at the request of the Government of Iraq,” his statement said.
“As a result of the Coalition’s action, Daesh has lost more than 98 per cent of the territory it once occupied in Iraq and Syria, and 7.7 million people have been liberated from its rule.”
Mr Williamson continued that such civilian deaths “remind us of the consequences of conflict and of the heavy price that the people of Syria have paid. It reminds us that when we undertake military action, we must do so knowing that it can never be completely without risk”.
He added: “Such incidents will not weaken our resolve to defeat Daesh and rid the world of its poisonous ideology of hate and intolerance. The UK’s commitment to the Global Coalition against Daesh and to the people of Iraq and Syria will remain as strong as ever.”