"I detect an undercurrent of self-doubt among some Americans about what your role in the world should be," the 66-year-old leader said in a speech to a joint session of Congress, which he delivered in English.
"This self-doubt is arising at a time when our world is at history's turning point. The post-Cold War era is already behind us, and we are now at an inflection point that will define the next stage of human history," he said.
The international order that the U.S. worked for generations to build is facing new challenges from those with values and principles very different from the U.S. and Japan, he said, pointing to China, Russia and North Korea.
"I want to address those Americans who feel the loneliness and exhaustion of being the country that has upheld the international order almost single-handedly. I understand it is a heavy burden to carry such hopes on your shoulders," he said.
"The U.S. should not be expected to do it all, unaided and on your own," he said.
"As the United States' closest friend, tomodachi, the people of Japan are with you, side by side, to assure the survival of liberty," the prime minister added.
"On the spaceship called 'Freedom and Democracy,' Japan is proud to be your shipmate. We are on deck, we are on task. And we are ready to do what is necessary," he said to big applause.