Skipper Paul Robinson's testimonial match will see Millwall take on Rayo Vallecano at The Den next month.
The Madrid-based side, who finished eighth in La Liga last season, will be the opponents on Saturday, July 27.
"I'm delighted to have this fixture for my testimonial," Robinson told the club's official website.
"Rayo are a very good side and will provide us with an extremely competitive outing the week before the Championship season kicks off.
"Personally, I am working very hard on my fitness so that I can play a major role, not only in this game but in a very important season coming up for the club as well."