Yoon's fate has been hanging in the balance after the court defied expectations of a swift decision on whether to oust him for violating his constitutional duty or reinstate him, instead deliberating for more than five weeks in tight secrecy.
The court is scheduled to deliver its ruling at 1pm AEDT in a nationally televised session.
Yoon is not expected to attend, but if he is stripped of his presidency, an election must be held within 60 days. If his powers are restored, Yoon faces the challenge of helping Asia's fourth-largest economy navigate the fallout from the stiff tariffs imposed by the United States, its main security ally.
The ruling will cap months of political turmoil that have overshadowed efforts to deal with the new administration of US President Donald Trump at a time of slowing growth.
Separately, 64-year-old Yoon faces a criminal trial on insurrection charges. The embattled leader became the first sitting South Korean president to be arrested on January 15 but was released in March after a court cancelled his arrest warrant citing questions over the procedural legitimacy of the investigation.
The crisis was triggered by his December 3 declaration of martial law, which he has said was needed to root out "anti-state" elements and the opposition Democratic Party's alleged abuse of its parliamentary majority that he said was destroying the country.
He lifted the decree six hours later after lawmakers defied efforts by the security forces to seal off parliament and voted to reject it. Yoon has said he never intended to fully impose emergency military rule and tried to downplay the fallout saying nobody was hurt.
South Korean parliamentary lawyers seeking Yoon's removal compared him to a dictator during the trial at the Constitutional Court, which currently has eight judges with one seat vacant. Under the Constitution, at least six of them must agree for Yoon to be removed.
The wait of more than a month for the decision has exacerbated tensions between the ruling party, with many of its members calling for Yoon's reinstatement, and the opposition, which has said his return would deal a serious blow to the country's constitutional order.
It has also deepened division among the public, with large crowds packing Seoul streets in rallies for and against Yoon's removal, and increasingly heated rhetoric.
Police are bracing for potential violence and plan to mobilise more than 14,000 officers in the capital.
The deep divisions suggest Friday's ruling will do little to ease the tension, analysts have said.
Experts also question whether Yoon, who has been badly bruised by personal scandals and political feuds, would be effective in the remaining two years of his term even if his powers were restored.
The majority of the public backs Yoon's removal, according to opinion polls, although support has ebbed since the initial days after the martial law declaration.
A Gallup Korea poll released last week showed 60 per cent of the respondents saying Yoon should be ousted.
