The 40-year-old suspect, a Spanish national of French origin, was located during a routine inspection in Seville on 25 February.
This is the second arrest in the investigation, after the police detained a 27-year-old man of Moroccan origin in February.
It was discovered that he had acted as an accomplice and look-out, while the new detainee attacked the victim in order to steal his belongings.
Father Gross was killed by suffocation soon after he arrived at his tourist accommodation in Calle Viento in Malaga city centre.
The priest had planned to stay for a week and had booked a boat trip along the Costa del Sol.
Although the police quickly identified the killer, they had been unable to locate him until now.
The court had issued a European arrest warrant on suspicion that he had fled abroad.
Father Gross was a loved member of the Jesuit community in Boston.
He had also taught history and science at a Massachusetts high school and at a university for years.