The Wales international made nine appearances for the Lions during his loan from Crystal Palace and scored once - against London rivals Charlton.
Easter's form impressed former Millwall manager Kenny Jackett, who initiated the deal as one of his last acts before leaving The Den earlier this month.
The 31-year-old was part of the team which reached the FA Cup semi-final before losing to eventual winners Wigan at Wembley.
Easter said: "The circumstances are slightly strange I suppose that Kenny has left the club now, but when I was on loan myself and Kenny agreed everything and I was expecting to be playing for him next season.
"However I've been in the game long enough to know how football works, so will now be looking to do well under the new manager.
"There were some highs and lows in my time at Millwall last season, obviously scoring on my debut at Charlton was great, but after the FA Cup semi-final the form wasn't great and going into the last game of the season needing to get something or for other results to go our way.
"Next year we want to enjoy our football a bit more and be pushing up the table, not be involved at the bottom. Now, however, I'm looking forward to the new season and playing under whoever the new manager is."