Human Rights Watch (HRW) said authorities arrested Samar Badawi and Nassima al-Sadah, who campaigned for women's right to drive and the lifting of the male guardianship system, in the past two days.
“The arrests of Samar Badawi and Nassima al-Sadah signal that the Saudi authorities see any peaceful dissent, whether past or present, as a threat to their autocratic rule," said Sarah Leah Whitson, Middle East director at Human Rights Watch, in a statement.
More than a dozen women's rights activists have been targeted since May. Most campaigned for the right to drive and an end to the kingdom's male guardianship system, which requires women to obtain the consent of a male relative for major decisions.
Badawi has received the United States' International Women of Courage Award in 2012 for challenging the guardianship system, and was among the first women who signed a petition calling on the government to allow women to drive, vote and run in local elections.
She is also the sister of Raif Badawi, a prominent human rights campaigner, who was sentenced to 10 years in prison in 2014 on charges of insulting Islam on his blog.
Sadah, from the restive Shia-majority Qatif province, has also campaigned to abolish the guardianship system and the right to drive. She was a candidate in the 2015 local elections which saw women run in elections for the first.
Her name was ultimately removed by authorities, the rights group said.
Crown Prince Mohammed bin Salman has courted Western allies to support his economic reform plan, offering billions of dollars of arms sales and promising to fight radicalism in the kingdom.
Hundreds of billions of dollars of investments were discussed during his trips to the United States and Europe.
In May, authorities arrested 10 women's rights activists, including Eman al-Nafjan, Loujain al-Hathloul, Aziza al-Yousef, Aisha al-Manea, Ibrahim Modeimigh and Mohammed al-Rabea.
Officials said seven people had been arrested for suspicious contacts with foreign entities and offering financial support to "enemies overseas", and that further arrests could be carried out as the investigation proceeded.
In June, the government ended a decades-old ban on women driving cars as part of a bid to diversify the economy away from oil and open up Saudis' cloistered lifestyles.