The Austrialian custodian had already been offered a two-year deal with Stockport County but decided to try his chances at Millwall instead.
Milosevic was signed from Perth Glory by Leeds United for £110,000 but often found himself behind Nigel Martyn and Paul Robinson in the pecking order at Elland Road Opportunities were few and far between but he enjoyed successful loan spells with Plymouth Argyle, Wolves and Crewe Alexandra.
"It's an opportunity that is too good to pass up," revealed Milosevic.
"I've been training for the past two months determined to get myself in peak condition for the pre-season. The Stockport offer was flattering, but the opportunity to go to a First Division club is just too good to turn down. I believe in my ability and I'm confident that if I'm given a chance at The Den I'll show Dennis Wise that I would be an asset to his team next season.
"I know there's quality keepers at Millwall; lads that I admire. But I do believe I can be first-choice there if given the chance to show what I can do. Hopefully, it'll work out well for me at the end of the month."