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. She was not engaging in any work as such, but was meeting with friends she had made over the years", said FMM spokesman Sunil Jayasekera.
There was no immediate comment from immigration officials or from the Brussels-based IFJ which has supported Sri Lanka's media activists for years.
Local journalists have also been victims of crimes and many say they practise self-censorship.
An anti-establishment editor had fled the country in August, less than a month after armed men held her at knifepoint.
In 2009, the Sunday Leader newspaper's editor Lasantha Wickrematunge, a staunch critic of the government, was shot dead near his office just outside the capital.