Foreign currency assets (FCA), which form a key component of reserves, rose by $3.54 billion to $377.773 billion. FCAs are maintained in major currencies like US dollar, euro, pound sterling and Japanese yen.
Buoyed by overnight gains on Wall Street, Tokyo's Nikkei began the first day of trading in 2018 by rallying more than 2 percent. Crude oil prices were at 2-1/2-year highs, helping fuel inflation expectations.
The Australian dollar rose 0.1 percent to $0.7586 against the bogged down dollar, putting a bit of distance between a five-month trough of $0.7532 plumbed overnight on dovish-sounding Reserve Bank of Australia policy meeting minutes.
Although Prime Minister Shinzo Abe's ruling party is expected to maintain a solid majority, the memory of recent election upsets in the United Kingdom and elsewhere has kept investors cautious. The dollar fetched 112.22 yen, little changed on the day.
The dollar's global weakness was mostly an offshoot of President Donald Trump's threat, who said he was willing to risk a government shutdown to ensure funding for a wall along the US-Mexico border, analysts said.