Cengiz Yalcin had been arrested in August in Spain along with two Chechen suspects who the Spanish interior ministry said were Al-Qaeda members plotting an attack in Europe.
The three were released on bail by a judge in Madrid in March pending terrorism charges. Yalcin was arrested in Gibraltar , authorities said.
Spanish media at the time cited police sources saying the three were suspected of plotting to drop explosives on a shopping mall in Gibraltar during the 2012 London Olympics.
The governor's office in Gibraltar, a British-ruled enclave on the southern tip of Spain, said in a statement that Yalcin was arrested as a "prohibited immigrant".
A police spokesman who asked not to be named told AFP that Yalcin was deported to Spain as an undocumented immigrant -- not due to any arrest warrant.
"He's got no documentation which allows him to be in Gibraltar," the spokesman said.
After the three suspects were arrested in August, the Spanish interior ministry released a video found in Yalcin's home which they said showed a trial bomb attack using a model aircraft.
Police found explosives in his home where he was arrested in the Spanish city of La Linea de la Concepcion, near the border with Gibraltar, the government said at the time.
The judge in Madrid's National Court is investigating terrorism and explosives charges against the three suspects but ruled in March that there was not yet enough evidence to keep them in custody.
Yalcin's lawyer said at the time of his arrest that his client was an engineer with an international company who had lived for seven years in Gibraltar and had a British work permit.