Mohammed al-Otaibi, 49, fled to neighbouring Qatar in March after he had faced in Saudi Arabia charges related to his human rights work and was referred to an anti-terrorism court, the Gulf Center for Human Rights said in a statement.
"At dawn, on 28 May 2017, Otaibi and his wife were forcibly deported to Saudi Arabia while on their way to Norway," said the Gulf Center, which has offices in Copenhagen and Beirut.
It said Oslo had agreed to provide them with travel documents and the right to seek asylum as soon as they arrived.
The official Qatar News Agency (QNA) said that the deportation had been confirmed by a foreign ministry source.
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"The extradition was... Based on legal procedures and regional and international agreements relating to the extradition of accused persons and criminals," the agency said.
First arrested in 2009, Otaibi in 2013 co-founded the Union for Human Rights in Riyadh.
Authorities ordered it shut after about one month, but he continued his work, issuing reports and giving television interviews, the Gulf Center said.
New York-based Human Rights Watch warned in April that Otaibi would be at risk of a long prison sentence and possible ill-treatment if forcibly returned to Saudi Arabia.
Saudi Arabia says that human rights are a matter of definition and "values" from one country should not be imposed.
It says the kingdom has made great strides in rights to education, healthcare and other areas.
The move to deport Otaibi comes at a particularly sensitive time in relations between Saudi Arabia and Qatar.
Reports last week said Qatar's leader, Emir Sheikh Tamim bin Hamad Al-Thani, made explosive remarks on state media criticising Gulf policy towards Iran, essentially putting Doha at odds with Riyadh.
The incident has pushed relations between the two countries to the lowest level for several years.
Qatari news sites were subsequently blocked in countries across the region, including Saudi Arabia.
In a further sign of a deepening rift between the two countries, the Saudi newspaper Okaz has reported that members of a prominent Saudi family demanded Qatar's state mosque, the Sheikh Muhammad Ibn Abdul Wahhab Mosque, be renamed.
The demand came amid questions over the Qatari royal family's link to Abdul Wahhab, co-founder of the Saudi state.