The Lions all-time leading goal scorer was offered a new contract after notching up 10 goals this season, convincing manager Kenny Jackett that he still has something to offer, regardless of what division the club may be in next term.
"I am delighted to stay on," Harris told the club's official website. "My top priority is to try and get the goals to help Millwall back to the Championship.
"We are in a really strong position for the Play-Offs and still in with a chance of going up automatically. It will be frustrating sitting out the game against Walsall because I had my nose operated on this week after breaking it at Crewe last weekend, but I hope to back for Yeovil this Friday and am desperate to play a part in winning promotion here."