The 21-year-old has been snapped up by the Lions after being told he was not going to be offered a new contract at the Withdean.
"I'm delighted. I knew I had to get over the disappointment at Brighton quickly and this is a good move for me," he said.
"I heard from Brighton they weren't keeping me and then the same day or the day after I heard from Kenny Jackett at Millwall.
"The main interest came from when I played against them in my second game for Brighton. They saw me play a few more times but I didn't know anything about it until I was released by
"There was other interest but Millwall are a massive club with great ambition. I would have been stupid to have turned them down."