Andy Hall, 34, faces jail if convicted on lawsuits brought by Thai fruit processor Natural Fruit.
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. The migrant rights campaigner has accused the company of trying to divert attention from the report's findings through its legal action.
Leading European food giants have called for the charges to be dropped, while rights groups have condemned the defamation proceedings by a Thai food industry that has faced a battery of bad headlines over recent months.
Prosecutors called their first witness Tuesday as the trial, which is linked to an interview Hall gave to the Al-Jazeera television network, began.
Hall faces a criminal defamation charge and his passport has been confiscated by Thai authorities pending trial. He could be imprisoned for up to a year if found guilty.
More serious charges under the Computer Crimes Act -- which carry up to seven years in jail for each count -- are due to be heard later in September.
The fruit processor is also seeking $10 million in damages through a civil suit.
Hall has throughout stood by the report, accusing the company of trying to muzzle his findings.
"What I've revealed is just the tip of the iceberg... and they're cracking down," he told reporters before entering court.
"I don't believe there's any evidence against me to show that what I did was malicious or in some way against the company. I did it for the benefit of the workers," he said.
Hall, who was supported by a dozen labour rights activists in court, made his allegations in a report prepared 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' own-brand 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".
On Tuesday Natural Fruit chairman Virat Piyapornpaiboon rejected the accusations against his company.
"The report caused damage to me and my company. Any accusations were not true... If true, why are there so many workers who want to work at my factory?" he told AFP outside the court.
Thailand has long tapped 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.
In June the US State Department downgraded Thailand to its lowest ranking in a report on human trafficking, highlighting abuses in the fisheries industry among others.
Thailand draws tens of thousands of migrants -- often trafficked -- from neighbouring countries "who are forced, coerced, or defrauded into labour or exploited in the sex trade", said the report, which was released before the military seized power in May.
The new military regime triggered an exodus of Cambodian workers following the coup by threatening to arrest and deport illegal labourers, although thousands later returned as the junta regulated the labour market, simplifying the registration process for migrant workers seeking an official work permit.