Ali Hattar was detained and referred to the state prosecutor after an address he gave this month entitled Why We Boycott America.
The lecture was organised by the Committee For the Protection of the Nation and the Struggle Against Normalisation, a Jordanian group opposed to normalising ties with Israel which has had a peace treaty with Jordan since 1994.
Hattar is a committee member for the country's engineering union, the Jordanian Engineers Union, which supports moves to stop normalisation with neighbouring Israel.
The head of the union, Wail Saqqa, said: "Our lawyers have gone to see the prosecutor to understand the exact nature of the charges against Ali Hattar.
"These charges are still not clear but it seems they are accusing him of slandering a foreign country."
Saqqa said he heard the lecture and did not consider it to contain anything damaging.
"I only heard a realistic and historical presentation about what America is doing against oppressed people. It is occupying Iraq and backs the Zionists against our people in Palestine," Saqqa said.
"They may consider this to be offending to a foreign country but we see it as a nationalistic stand," he said.
Saqqa said Hattar's audience could have concluded that they should boycott American goods.