Japan is already Pakistan's biggest aid donor
Japanese Prime Minister Taro Aso has pledged up to $1bn (£670m) in aid to Pakistan, after talks with President Asif Ali Zardari in Tokyo.
Mr Aso said Pakistan's stability was important for the region and that of the international community.
Mr Zardari thanked Japan for supporting Pakistan's "fight against terrorism".
The pair were speaking on the eve of a donor conference hosted by the World Bank and Japan, which it is hoped will raise $6bn in loans and grant aid.
Almost 30 donor countries are to meet on Friday to pledge funds for the next two years.
The BBC's Roland Buerk in Tokyo says delegates will want to hear Pakistan's commitment to economic reforms and progress in its fight against an increasingly formidable Islamist insurgency.
Pakistan is also to outline its strategies to a separate ministerial-level session of the "Friends of Democratic Pakistan", which includes the US, Japan, China and Saudi Arabia.
Our correspondent says Pakistan is critical to the United States' hopes of stabilising Afghanistan, by defeating militants on both sides of the border.
Many Taleban fighters operate from Pakistan's lawless north-west. Militant groups have carried out suicide attacks and are increasingly challenging the writ of the state.
Pakistan narrowly averted a balance of payments crisis last year when it secured a loan of $7.6bn from the International Monetary Fund.
There is concern an economic meltdown in nuclear-armed Pakistan could fuel popular support for extremism.
Speaking in Tokyo, Pakistan's foreign minister said his country wanted a political expression of solidarity and support for its role as a frontline state fighting terrorism.