West Ham boss Alan Pardew believes the troubled star is ready to turn over a new leaf after moving to Upton Park from Newcastle.
Neither club has made mention of a price for the former Leeds and Charlton player, and it appears that Newcastle have simply been happy to get Bowyer's wages off their books.
Newcastle will receive a share of any fee West Ham receive, should Bowyer leave them. He arrives in east London with a warning to be on his best behaviour for West Ham.
Bowyer's career has been littered with indiscretions and controversy, but manager Pardew said: "He has taken a pay cut to join West Ham, and that says it all to me.
"He wants to play in the claret and blue because this is where he is from and he can see where this club is going. All those incidents that Lee has been involved with down the years have taught him lessons and everyone deserves another chance.
"I will not tolerate any other incidents, he now needs to have a clean bill of health. We had a fantastic disciplinary record last season - one sending-off all year speaks for itself. Lee needs to understand they are the standards we have set and I expect him to abide by them."
The 29-year-old midfielder, capped once by England, had a six-month stint with the Hammers in 2003 before moving to St James' Park.
He now returns to Upton Park and said: "I am desperate to do well for West Ham. I loved my time at Newcastle but this is my club. I didn't play well last time. I had a bad ankle injury and kept playing on it which was to nobody's benefit. I see this as unfinished business - I want to show the Hammers fans what I can do.
"When Alan Pardew came in for me there was never going to be any other outcome."
Glenn Roeder: "Lee did well for me for three months at the end of last season. I tried to persuade him to stay in the north, but he was extremely keen to move back to London, and in my experience, you do not get the best out of players in those circumstances.
"So I consider it the best thing to let him return to the south. There is no point in keeping any player who may be unhappy for whatever reason."