At 1513 GMT, the rand traded at 19.1425 against the dollar, about 1.1% weaker than its previous close.
The dollar index was trading up about 0.08% at 104.06, having rallied from a weak position earlier in the day.
The rand initially jumped on Wednesday after the annual budget speech, in which the finance minister said the government would tap 150 billion rand ($7.99 billion) from a central bank-administered contingency account in order to limit borrowing. But most of the gains had been reversed by the end of the day.
Its fall on Thursday was likely due to global rather than local factors, said Danny Greeff, an analyst at ETM Analytics.
"The extent of the rand's retreat is somewhat puzzling, but speaks to its lack of resilience to external shifts in market sentiment," he said.
"It is extremely vulnerable to broader market dynamics, with investor sentiment towards South Africa still weak."
The budget speech on Wednesday did little to suggest much-needed structural reforms, while political risk ahead of South Africa's upcoming election is keeping investors cautious, Greeff said.
On the stock market, the Top-40 index closed around 1.6% higher. South Africa's benchmark 2030 government bond was stronger, with the yield down 2 basis points to 10.005%.
(Reporting by Nellie PeytonEditing by Ros Russell and Frances Kerry)
