Ahmadinejad is to visit Cuba, Venezuela, Nicaragua and Ecuador in January.
U.S. Rep. Ileana Ros-Lehtinen, R.-Fla., chairwoman of the House Foreign Affairs Committee, described Ahmadinejad's itinerary as a "tour of tyrants."
She said Iran has defense and "other schemes" planned with "dictatorships" in Latin American that are meant to harm the United States.
"Iranian activity in the Western Hemisphere threatens regional security and stability," she said in a statement. "It is important that we join forces with other democracies in the region to counter and prevent further Iranian inroads in the hemisphere and root out the threat posed by Iran, its proxies, and others who seek to do our nation harm."
Venezuelan President Hugo Chavez and Ahmadinejad are strong allies. Chavez in recent statements speculated that his bout with cancer might be part of a plot by the United States to cripple Latin American leaders.
Ahmadinejad's trip comes as Western allies consider new sanctions on Iran for its nuclear program.