The left-back is off contract with League one outfit Bradford City after spending five seasons with the club.
Millwall secured promotion earlier this month after defeating Bradford 1-0 in the League One playoff in which Meredith featured for The Bantams.
Meredith had been hoping for a move to the Championship with several Championship clubs, including Leeds and Norwich, having previously expressed interest in signing the 29-year-old, who will complete a medical with Milwall when he returns from holiday next week.
Meredith joined Bradford City from York City in 2012, helping them get promoted into League One in 2013 and going on to make over 200 appearances for the club.
The fringe Socceroo, who made two appearances for Australia in 2015, was named in the PFA Team of the Year for the 2016-2017 EFL League One season.
Meredith joins a long line of Australians to ply their trade with Millwall including Tim Cahill, Kevin Muscat, Jason Van Blerk and Scott McDonald.
Bradford boss Stuart McCall was quick to offer Meredith a new contract to, but he elected to move on saying Millwall coach Neil Harris played a part in convincing him to join his club.
"I met Neil and he came across so well. He's ambitious and has big plans both for the football club and for me as an individual, which is exactly what you want to hear and made Millwall the most attractive option for me," Meredith told millwallfc.co.uk.
"I'm looking forward to meeting and getting to know my new teammates during pre-season. There's a lot of hard work ahead because the Championship is a very demanding League with tough games every week.
"I'm excited for what the future holds and can't wait to play as a Millwall player at The Den. Whenever I've played there before the atmosphere has been great."
Lions boss Harris said he was pleased to have secured his first signing since leading the club to the Championship.
"It's important for us now at Championship level that we strengthen positions across the park and James' arrival certainly does that. Competition for places is vital if we're to be successful," he said.
"He adds further quality to our left-hand side and arrives in form having enjoyed a brilliant season for Bradford. James has been putting in consistently good performances for some time, including against us, and I'm looking forward to working with him once pre-season training begins in the latter part of June."
