Zuma must come clean about his reasons for axing former Finance Minister Pravin Gordhan and his deputy, Mcebisi Jonas.
Vally said he would give reasons for his ruling early next week.
The judgement essentially means the president's hand has been forced, and reasons for his reshuffle and supporting documents must be submitted within five working days. The presidency also has to foot the bill for the DA's court costs.
Speaking to HuffPost SA, DA chairperson James Selfe said the ruling is not only a victory for the party, but for all South Africans.
"The president's decision to axe the finance minister and reshuffle his Cabinet affected the economy adversely. This ruling now forces the president's hand," Selfe said.
"He [Zuma] must carefully think about the reasons he is going to give us because he will be held to them."
But political experts are not convinced.
Professor of political science at the University of Western Cape, Bheki Mngomezulu, said he was surprised by the court's judgement.
"In the nature of the Constitution, the president has the right and the privilege to appoint and dismiss anyone in his Cabinet. If you look at the nature of government, you have three arms in the Executive, the Legislature and the Judiciary. No arm of government should interfere with another," Mngomezulu said.
"What the president did was not illegal or unconstitutional. The court is interfering with another arm of government."
Constitutional law expert Pierre de Vos instead said the court did nothing more than rely on the constitutional jurisprudence of the last 20 years.
"Every exercise of public power can be reviewed by a court on the basis of irrationality. For a court to know whether a decision is irrational, they need to know the reasons as to why the decision was made," de Vos said.
However, de Vos said it is unlikely the court's judgement will deal a blow to Zuma.
"After the reasons for the Cabinet shuffle are given by the president, it must be decided on what basis are the reasons irrational. It will be almost impossible for the court to rule against the president, unless there is evidence of corruption or bad faith in his decision-making," de Vos said.
"The president has the privilege of reshuffling his cabinet. How do you prove the decision was made in bad faith?"