A terrorism court convicted Nourah bint Saeed al-Qahtani of "using the internet to tear the social fabric" and "violating public order by using social media", rights group Dawn reported.
Little is known about her other than that she criticised Saudi leaders.
Another woman was jailed for 34 years over her Twitter activity on 9 August.
Abdullah Alaoudh, director of research for the Gulf region at Dawn, told the BBC's Newshour programme that the US-based group had learnt of Qahtani's case only after being passed court documents by a judicial source, and that it was continuing to investigate.
"Nothing in her court documents pertains to any violence or criminal activity," he said.
"The charges against her are really broad. They are using the counter-terrorism law and the anti-cybercrime law... that can criminalise any posting that is even remotely critical of the government," he added.