South Africa announced in October that it would be pulling out of the court following a dispute over the failure to arrest Sudanese President Omar al-Bashir, who visited the country in 2015 despite an arrest warrant from the ICC over alleged war crimes.
"The cabinet decision to deliver the notice of withdrawal... without prior parliamentary approval is unconstitutional and invalid," said Judge Phineas Mojapelo in the North Gauteng High Court.
The president and ministers, the judge added, "are ordered forthwith to revoke the notice of withdrawal."
The ruling is being celebrated by the opposition Democratic Alliance (DA), one of the groups who brought the case before the court.
"The withdrawal by the South African government from the ICC was irrational and unconstitutional," DA lawmaker James Selfie told AFP.
"We would like South Africa to stay in the ICC because we believe that it is consistent with our constitution and with the legacy of Nelson Mandela," he added.
The role of the ICC in Africa is a contentious one as currently nine out of the court`s ten investigations concern African countries. However many of the investigations were referred to the ICC by the governments of those states, who are also signatories to the court.
The ruling also comes just three weeks after African Union (AU) leaders stated that individual AU countries should leave the ICC in favor of the African Court of Justice.
The court ruling is a setback for President Jacob Zuma`s government, which could appeal.
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