It was the latest arrest of a Singaporean for allegedly supporting the jihadists or planning to fight with them, with the city-state's leaders warning it is a prime target for an attack due its status as a regional financial centre.
Munavar Baig Amina Begam, 38, a naturalised Singapore citizen originally from India, was radicalised online by a contact who convinced her the ISIS was fighting to defend Sunni Muslims in the conflict zone, the Ministry of Home Affairs said in a statement.
"She was radicalised to the extent that she was prepared to undergo military training and take up arms to fight for ISIS in the Middle-East if called upon by ISIS to do so," said the ministry, using another name for the jihadists.
Amina, who also shared ISIS propaganda on social media, has been detained under the tough Internal Security Act, which allows for detention without trial for up to two years.
A religious school teacher, Abu Thalha bin Samad, was also detained under the act for being a member of Southeast Asian militant group Jemaah Islamiyah (JI), the ministry said.
JI was behind a series of deadly attacks in the 2000s, including the 2002 Bali bombings, but has not carried out major assaults in recent years.
The 25-year-old Singaporean had been teaching in a JI-linked school outside the city-state. The ministry worked with a regional government to deport Abu Thalha to Singapore before he was detained but did not say which one. JI is mainly active in Indonesia.