Andy Hall, 34, faces civil and criminal lawsuits submitted by Thai fruit processor Natural Fruit, after his report levelled accusations of forced and child labour, unlawfully low wages and long hours at one of its factories.
Natural Fruit is a major supplier to the European drink market and leading European food companies have urged the company to drop its legal action.
Prosecution witnesses are expected to begin testimony on Tuesday as the trial, which is linked to an interview Hall gave to the Al-Jazeera television network, gets under way.
Defamation is a criminal offence in Thailand and Hall, who has had his passport confiscated by Thai authorities pending the trial, could be jailed for up to a year if found guilty.
More serious charges under the computer crime act - which carries up to seven years in jail for each count - are due to be heard later in September.
Natural Fruit is also seeking $10 million through a civil suit.
Hall has stood by his report, labelling the charges against him "judicial harassment" in a nation whose image has been battered by a slew of recent scandals linked to the treatment of migrant workers.
"This is a country that's dependent - 10% to 15% of the labour force is from overseas - and they've had no migration policy, no rational policy, no long-term policy," he told AFP ahead of the trial.
"The exploitation has been getting worse and more confused and more messy. The corruption is systematic. We haven't seen anyone addressing that... It is just the tip of the iceberg."
Hall made the allegations in a report last year for Finnish rights watchdog Finnwatch.
A Natural Fruit factory in southern Thailand was investigated for a Finnwatch report called "Cheap Has a High Price" because it produced pineapple concentrate for Finnish supermarkets' private label products, according to the watchdog.
Finnwatch has called on Thailand, the world's largest pineapple producer, to change its approach "instead of issuing threats and exploiting workers".
Thailand taps the labour markets of poorer neighbours Myanmar, Cambodia and Laos, whose migrants help keep major Thai industries from seafood to construction afloat.
But they often lack official work permits and are paid below the minimum wage.
Thailand's junta triggered an exodus of Cambodian workers following May's military coup with its threat to arrest and deport illegal labourers, although thousands have since returned.
The junta has since pledged to simplify the registration process for migrant workers seeking an official work permit.