She was the latest in a series of suspected Isis sympathisers detained in Spain since last year over security fears.
Police said she was suspected of recruiting other women via the Internet to the group, also known as Daesh, which has claimed numerous kidnappings and executions.
At the time of her arrest early Saturday in the coastal town of Gandia near Valencia, she "was making the last preparations for her journey" to Syria, a police statement said.
She was not the first woman arrested in Spain for allegedly recruiting women and girls for the armed group, after another was detained on the island of Lanzarote in July.
More than 100 people from Spain are suspected of having joined jihadist fighters in Iraq and Syria, and authorities fear they may return to launch attacks.
In March 2004, Al-Qaeda inspired bombers blew up four packed commuter trains in Madrid, killing 191 people.
More than 20 people including various Moroccans were convicted of that attack.
Police said the woman detained Saturday was Moroccan but had lived for a long time in Spain.
Officers arrested her "for suspected links with jihadist terrorist activities".
"She spread jihadist ideology on the Internet, justifying terrorist acts and spreading videos that glorified executions of people," it said.
"She had focussed her online activity on recruiting women to go and swell the ranks of Daesh."