The man is "suspected of teaching some of those arrested in this case about communication tools involving new technology and of training them in marksmanship", Morocco's central office for judicial investigations said in a statement.
The counter-terror organ added he subscribed to "extremist ideology" and also has Spanish citizenship.
The ongoing investigation into the double murder uncovered the man was involved in the "recruitment of Moroccans and sub-Saharans to carry out terrorist plans in Morocco", the statement said.
Danish student Louisa Vesterager Jespersen, 24, and 28-year-old Norwegian Maren Ueland were found dead at an isolated hiking spot in the High Atlas mountains, south of Marrakesh on December 17.
The two women were beheaded, authorities have said.
Ahead of Saturday's arrest, Moroccan authorities had previously arrested 18 people for alleged links to the murders.
The four main suspects were arrested in Marrakesh and belonged to a cell inspired by Islamic State group ideology, Morocco's counter-terror chief Abdelhak Khiam told AFP this week.
But none of the four had contact with IS members in Syria or Iraq, he said.
The head of the suspected cell is 25-year-old street vendor Abdessamad Ejjoud, according to investigators.
He was identified in a video filmed a week before the double-murder, in which the four main suspects pledged allegiance to IS leader Abu Bakr al-Baghdadi, according to authorities.
The killings have shaken Norway, Denmark and Morocco. Another video circulated on social networks allegedly showed the murder of one of the tourists.
Morocco, which relies heavily on tourism income, suffered a jihadist attack in 2011, when a bomb blast at a cafe in Marrakesh's famed Jamaa El Fna Square killed 17 people, mostly European tourists.
An attack in the North African state's financial capital Casablanca killed 33 people in 2003.