Chee Soon Juan, 45, secretary general of the Singapore Democratic Party (SDP), remains free pending his filing of an appeal by Wednesday.
Supreme Court Justice Belinda Ang Saw Ean also sentenced Chee's sister, SDP member Chee Siok Chin, to 10 days in jail for contempt. She is also free pending appeal.
Chee, his sister and their party have already been found guilty of libelling Singapore's founding father Lee Kuan Yew and his son, Prime Minister Lee Hsien Loong.
The contempt proceedings stemmed from comments they made during a three-day hearing before Ang last week to determine libel damages.
She ruled that Chee's behaviour "amounted to acts that scandalised the court" and adversely affected the administration of justice.
Chee is among a very few in Singapore to have spoken out against the People's Action Party (PAP), which has ruled since 1959.
In 2006 he was jailed for a then-unprecedented eight days for contempt after questioning the integrity of the judicial system in a statement he read during his bankruptcy hearing earlier that year.
He was declared bankrupt after failing to pay 500,000 Singapore dollars (US$366,300) in libel damages to Lee Kuan Yew and another former prime minister over remarks made in 2001.