Spanish state radio said its correspondent Fran Sevilla, who reported the troops' withdrawal from their base at Diwaniya earlier on Friday, was in good health.
He was reportedly held by a group loyal to Shia leader Muqtada al-Sadr for four hours.
Earlier on Friday, Sevilla reported from Diwaniya that the last of the Spanish troops had left their base and were on their way to Kuwait. He said he was the last Spaniard left in the south-central Iraqi city.
The soldiers were among the last remaining in Iraq from a force of up to 1400 sent by the previous pro-American government. They had been stationed in a largely Shia Muslim area of south-central Iraq, including the cities of Najaf and Diwaniya.
Spain's month-old Socialist government - elected three days after the 11 March train bombings killed 191 people in Madrid - has been withdrawing the troops, fulfilling a campaign promise made before the railway attacks.
Defence Minister Jose Bono said the withdrawal would be completed by 26 May, but officials have said it will over by Friday night.
The United States had urged Spain to reconsider the pullout but Prime Minister Jose Luis Rodriguez Zapatero, who has called the war a fiasco, would not be moved.
"What strengthened the stance of those calling for troop withdrawal was the information... that US occupation forces attempted to coax or to force Spanish troops into arresting Shia leader Muqtada al-Sadr"
Aljazeera's Spain correspondent
On Thursday, Spanish troops in the process of withdrawing came under attack from suspected Iraqi anti-occupation fighters.
One attacker was killed and another seriously wounded and one Spanish soldier was slightly wounded in the attack.
Aljazeera's correspondent in Spain, Tayseer Alouni, said the withdrawal of Spanish troops has been warmly welcomed in the country.
"What strengthened the stance of those calling for troop withdrawal was the information ... that US occupation forces attempted to coax or to force Spanish troops into arresting Shia leader Muqtada al-Sadr," he said.
"But the Spanish troops rejected these US attempts ... repeatedly resisting their demands."