Millwall will play a Portuguese side in their only pre-season friendly ahead of the 2015/2016 season.
They will welcome CD Tondela - winners of Segunda Liga - to The Den on August 1.
The side secured promotion to the Primeira Liga for the first time in their 82-year-old history, while Millwall will embark on a League One following their relegation from the Championship.
The clash is expected to round off the Lions' pre-season fixture list and will follow away games at Bromley on July 14, AFC Wimbledon on July 18 and Stevenage on July 25.
Kick off will be at 2pm with ticket details announced in due course.