He said the plan would not affect the government's ability to cut taxes and was the "right thing to do" in an increasingly dangerous world.
Sunak has committed to spending 2.5% of GDP on defence by 2030.
However, the government's numbers have faced scrutiny from economists who have warned they would lead to bigger spending cuts in other areas.
Asked at a press conference in Berlin whether his defence increase will mean tax rises and deep cuts, he said that was "not a fair characterisation".
"We have made a choice and I am not shying away from that choice," he said. "All governing is about prioritising. I have decided to prioritise defence because I think that is the right thing to do for our country."
Sunak said investment in the NHS and schools which along with international development are protected areas - was "not going to change".
He insisted the plan, which would cost £4.5bn a year by 2028-29, was fully funded and has already ruled out any increase in borrowing or debt to pay for it. Of the increase, £1.6bn will come from an increase in R&D funding for defence, although it was unclear which department would miss out as a result.
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