WASHINGTON—The U.S. has “serious concerns” with proposals being discussed in the global rewrite of corporate tax rules, Treasury Secretary Steven Mnuchin said, as he introduced a new idea that could complicate a deal over adapting the tax system for the digital economy.

In a letter to the Organization for Economic Cooperation and Development, Mr. Mnuchin cited concerns raised by U.S. businesses. He said that changing the mandatory rules for when countries have the right to tax companies could affect “longstanding pillars...