Manager Ian Holloway stated in February that he would be offering the 32-year-old a new deal, with terms finalised to keep him at The Den several weeks ago.
"Jermaine lifts our group whether he comes off the bench or not," Holloway said when first confirming that he wanted to keep hold of the Welsh international.
"His training is something that is completely and utterly outstanding. I let Jermaine leave Palace because I couldn't guarantee him a new deal.
"I liked him but somebody else didn't. I am giving him a new contract because the whole atmosphere at Palace dipped after he left. I've already agreed to give him another year."
Jermaine Easter has played for 10 other clubs as well as Millwall, having started his career at Hartlepool United.