"Today, I've decided to declare Francisco Martinez ... who works for the United States embassy, persona non-grata," Morales, a fierce critic of the United States, said at a press conference in La Paz.
The president said he needed to expel Martinez "to put an end to a foreign conspiracy."
Morales, who often calls the United States "the empire," previously ejected the U.S. ambassador and U.S. anti-narcotics officials from the U.S. Drug Enforcement Administration.
A spokesman at the embassy said Martinez was a mid-level official at the diplomatic mission, and that his title was second secretary. The embassy made no other comment.
Last month, Morales accused Washington of plotting against the state-run energy company, YPFB, a claim the U.S. government dismissed.
Morales expelled the U.S. ambassador from Bolivia last year, charging he was conspiring, accusations that the United States called "false" and "absurd."
Earlier this year, Ecuadorean President Rafael Correa expelled a U.S. Embassy official that he said was the director of the CIA in Ecuador.
Correa is a close ally of Morales, as is Venezuelan President Hugo Chavez, who expelled the U.S. ambassador to his country last year.
All three of the leftist leaders have said the U.S. meddles in their domestic politics.
