"Market forces are driving the exchange rate," Abid Qamar, the central bank spokesman, told Reuters via text message.
The State Bank of Pakistan (SBP) usually makes an announcement about devaluations a few hours after they take place.
Pakistan's economic fundamentals have deteriorated ahead of the country's July 25 general election, with most financial analysts expecting the next government will need to seek a post-election bailout from the International Monetary Fund (IMF).
Pakistan's current account balance has been widening sharply, while foreign currency reserves have plummeted over the past few years.
The central bank last devalued the rupee by about 4 per cent in June, after weakening the currency by about 10 per cent during interventions in March and December.