The International Federation of Journalists (IFJ) representative for the Asia-Pacific region, Jacqui Park, was taken into custody by immigration officials, the Free Media Movement (FMM) said.
"She was meeting with us at a hotel in Colombo while on holiday," said FMM spokesman Sunil Jayasekera. "She was not engaging in any work as such, but was meeting with friends she had made over the years.
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There was no immediate comment from immigration officials or from the Brussels-based IFJ which has supported Sri Lanka's media activists for years.
Sri Lanka is known to have blacklisted many foreign journalists over their reports on the country's human rights record and alleged war crimes in the final stages of the ethnic war in 2009.
Local journalists have also been the victims of crimes and many say they practise self-censorship.
An anti-establishment editor fled the country in August, less than a month after armed men held her at knifepoint.
In 2009 the Sunday Leader newspaper's editor