Darren Byfield has agreed a three-year deal with Millwall after bidding a fond farewell to their League One rivals Gillingham.
The 29-year-old striker has ended a two-year stay at Priestfield, where he scored 20 goals in 58 starts.
Millwall have pursued Byfield since last summer, and they made another attempt to bring him in during the January transfer window.
The offer of a three-year contract is believed to have been a key factor in Byfield's decision to move to The Den.