ATLANTA—The U.S., Japan and 10 other countries around the Pacific reached a historic accord Monday to lower trade barriers to goods and services and set commercial rules of the road for two-fifths of the global economy.

For the U.S., the Trans-Pacific Partnership agreement opens agricultural markets in Japan and Canada, tightens intellectual property rules to benefit drug and technology companies, and establishes a tightknit economic bloc to challenge China’s influence in the region.

President...