Sunderland officials were adamant they would only allow Patterson to leave Wearside this month if they were able to secure a new goalkeeper of their own.
They achieved that aim earlier today, wrapping up the £3m signing of Swedish shot-stopper Melker Ellborg from Malmo.
Ellborg's arrival enabled the Sunderland hierarchy to give the green light to Patterson's proposed switch to Millwall, and having travelled to London to complete a medical, the 25-year-old has now completed his switch to the Championship promotion hopefuls.
Patterson has made 169 senior appearances for Sunderland and played a pivotal role in last season's successful promotion from the Championship.
However, he lost his place as Regis Le Bris' number one goalkeeper when Robin Roefs signed in the summer and has not played a single minute of Premier League football this season.
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Sunderland sporting director, Kristjaan Speakman, said: "Anthony has been a standout performer for us in recent seasons, and it's important for him to maintain regular playing time at this stage of his career.
"Although Robin's quick transition and impressive performances have reduced his opportunities, he's continued to train at a high level, and this loan provides the necessary platform for him to continue developing his game."
Millwall have had a busy weekend as they have also completed the loan signing of Watson from Brighton.
The winger joined Brighton from Sunderland in the summer, having scored the winning goal in last season's play-off final at Wembley. He has since made ten senior appearances for the Seagulls.
Brighton head coach Fabian Hurzeler said: “Tommy’s settled in well, but has had a frustrating time with injuries, but he’s now fit and keen to play and this gives him the opportunity.
“He is an exciting young player and he now has the chance to help Millwall with their Premier League promotion push.
“The move will really benefit his development. It’ll give him significant game time and allow him to build on an outstanding breakthrough season with Sunderland last season, where he was integral in their promotion.
“We wish him well for the second half of the season and look forward to welcoming him back in the summer.”