The six – aged 20 to 29 – were both Danish and foreign nationals living in the Scandinavian country, prosecutor Lise-Lotta Nilas said, describing them as “fighters.”
The six have denied the charges. They face at least four years in prison if convicted.
The suspects joined so-called Islamic State between 2013 and 2016, the prosecution statement said.
Danish police have for the past year conducted an investigation after international partners passed on a list of names of people suspected of having links with Islamic State.
“The six were on an Islamic State membership list that was seized,” deputy prosecutor Anders Riisager told news agency Ritzau.
The trials were to be held at different locations, including in the capital Copenhagen and Aarhus, western Denmark.
Last month, a Danish appellate court stripped a man of his Danish citizenship for volunteering to fight for Islamic State in Syria.
Danish-born Hamza Cakan, who has dual Danish and Turkish citizenship, was also to be deported to Turkey after serving a six-year prison sentence, the Eastern High Court said.