Christine Lambrecht warned Germany that it must keep spending high enough to meet its Nato commitment — 2 per cent of GDP — in the long term.
Her declaration, coupled with Germany's supplying weapons to Ukraine, represent a major shift in the country's military ambition.
Days after Russia's invasion of Ukraine in February, Chancellor Olaf Scholz announced a €100 billion ($101bn) special fund to lift military spending to more than 2 per cent of GDP.
Ms Lambrecht said on Monday that after the special fund was spent, Germany must accept a military leadership role and carry on with the 2 per cent promise in the long term.
In a speech outlining her policy goals, Ms Lambrecht insisted the money would have to come from a ramped-up defence budget.
“We need this money without ifs or buts, and we need it for the long run so that the effort we are making with the 100 billion is not in vain,” she said.
“We must prevent a situation where, in a few years, we cannot afford the maintenance of the equipment that we are purchasing now."
Germany's size, geographic location and economic strength “bestow upon us a leadership role, regardless whether we like it or not — including in the case of the military,” she added.
The fund is meant to bring the German military’s weapons and equipment up to standard after the end of the Cold War and decades of attrition.
Nato allies have criticised Berlin strongly in the past for not reaching the 2 per cent target.
Ms Lambrecht also called for a softening of Germany's strict military export rules to enable joint European defence projects.
That could antagonise members of her own Social-Democrat party as well as the Greens in the governing coalition.
At the same time, she rejected Ukrainian demands to supply Kyiv with tanks.
“No country has delivered Western-built infantry fighting vehicles or main battle tanks so far,” Ms Lambrecht said.
The war in Ukraine has also put German — and European — energy supplies into focus, with many reliant on Russian gas which can be switched off.
Germany is well-positioned to get through the winter despite reduced gas flows from Russia thanks to measures including extending two nuclear power plants' lifespans, Chancellor Scholz said on Tuesday.
Germany's gas storage levels are higher than this time last year and will continue to be filled before winter, he said.
He added that he wanted to hurry through reforms to the energy market to ease steep rises in the price of electricity before the coming European winter.
“We will now push this through with great speed so that we can relieve the burden on consumers and companies,” Mr Scholz said. “This will happen at the speed necessary to get it under control this winter.”
At the same time, Germany's economic outlook has “dramatically worsened” and the economy could stagnate or contract in the next half-year, an economy ministry report said on Tuesday.
A rising number of companies are going insolvent but there is no “insolvency wave” as such, the report said. It added that Germany's labour market had defied global uncertainties for the time being and that demand remains high.