General Qassem Soleimani, the head of Iran’s elite Quds Force, was killed in a strike near Baghdad airport by US forces.
His death marks an escalation of tensions between the US and Iran, which have been at boiling point since Donald Trump withdrew from the 2015 nuclear deal and imposed crippling sanctions on Tehran.
Iran has vowed “harsh retaliation” for the death of their general.
Speaking on Friday morning, Mr Corbyn condemned the move by the US.
He said: “The US assassination of Iranian general Qassem Soleimani is an extremely serious and dangerous escalation of conflict in the Middle East with global significance.
“The UK Government should urge restraint on the part of both Iran and the US, and stand up to the belligerent actions and rhetoric coming from the United States.
“All countries in the region and beyond should seek to ratchet down the tensions to avoid deepening conflict, which can only bring further misery to the region, 17 years on from the disastrous invasion of Iraq.”
Mr Corbyn has faced criticism in the past for accepting around £20,000 in fees for appearing on Iran’s state TV station Press TV.
The channel in banned in the UK over its role in the torture of an Iranian journalist.
Foreign Secretary Dominic Raab also released a statement urging a de-escalation.
He said: “We have always recognised the aggressive threat posed by the Iranian Quds force led by Qassem Soleimani. Following his death, we urge all parties to de-escalate. Further conflict is in none of our interests.”
After news of Soleimani’s death broke, Mr Trump tweeted an image of an American flag.
The US Defence Department said it killed Soleimani because he “was actively developing plans to attack American diplomats and service members in Iraq and throughout the region”.
It also accused Soleimani of approving the attacks on the US Embassy in Baghdad earlier this week.
An adviser to Iran’s President Hassan Rouhani said: “Trump through his gamble has dragged the US into the most dangerous situation in the region.
“Whoever put his foot beyond the red line should be ready to face its consequences.”
The attack comes after a year of escalating tensions in the Middle East and represents a drastic change for American policy toward Iran.
In 2019 Tehran shot down a US military surveillance drone and seized oil tankers.
The US also blames Iran for a series of attacks targeting tankers, as well as a September assault on Saudi Arabia’s oil industry that temporarily halved its production.