The 30-year-old free agent becomes the Lions’ ninth addition of the summer transfer window.
Born in Australia, Dykes began his career in Scotland before joining Queens Park Rangers in 2020, where he established himself as a reliable Championship forward.
“I’m pleased to be signing Lyndon Dykes," said Steve Gallen, Millwall's Director of Football.
“He’s played at both Championship and international level, so he knows the standards required. He’s a genuine target man - physical, hard to knock off the ball, and someone who’ll give defenders a different problem to deal with. We’ve also seen how demanding this League can be, so he also adds some key depth in the attacking area.
“We’ve made a real effort to balance the squad with young, exciting talent alongside experienced heads who can guide them and help set the tone in the dressing room.
“I wish Lyndon the best of luck during this time with us.”
Dykes moved to Birmingham City in 2024 and helped the club secure promotion back to the Championship.
Millwall will now hope his experience and goalscoring ability strengthen their squad for the upcoming campaign.