The government has been accused by health experts, scientists and political rivals of tardiness in procuring vaccines against the new coronavirus - charges it rejects.
South Africa has suffered a surge in infections driven by a new, more contagious variant of the virus called 501Y.V2, which drove daily new cases to 21,000 in early January, although they have now eased to below 10,000.
"Ramaphosa wants a get-out-of-jail-free card by blaming the West for having ordered in time. We didn't order in time, and now we're blaming everyone else," said Helen Zille, chair of the DA's Federal Council, announcing the lawsuit.
The African National Congress government is already fighting a slew of court cases, including challenges to a ban on liquor sales to curb the spread of the virus and to a ban on the sale of cigarettes - which the government has lost but intends to appeal.
Zille said South Africans had a right to know where the government was procuring vaccines, how it would administer them, and who was benefiting from the process.
Lawyers for the DA had written to Ramaphosa, arguing that he had a constitutional obligation to be transparent about vaccine acquisition and distribution, and threatening litigation if he did not release details by Jan. 25.
A DA official said the court papers were still being prepared before being lodged at the Cape High Court on Thursday.
The government has said South Africa will get its first 1.5 million doses of AstraZeneca's vaccine from India's Serum Institute this month and next.
On Tuesday, Ramaphosa urged wealthy countries not to hoard surplus vaccines.
(Reporting by Wendell Roelf; editing by Promit Mukherjee and Kevin Liffey)