Bradley has made the temporary switch from Borussia Monchengladbach after impressing Villa boss Gerard Houllier during last summer's World Cup in South Africa.
He has become Villa's fourth signing of the transfer window after striker and ?18 million record signing Darren Bent, midfielder Jean Makoun and on-loan Tottenham defender Kyle Walker.
New man: Michael Bradley poses with the Aston Villa shirt after signing his loan deal
Bradley will provide competition for the likes of Nigel Reo-Coker, Barry Bannan, Makoun and Stiliyan Petrov in the centre of the park.
Houllier said: 'Michael is 23 and is a very hard-working midfielder who can score goals and is versatile.
'I saw him play four times in the World Cup last summer so, in terms of scouting, you cannot say the work has not been done because I saw him just a few months ago.
'We are very happy to have him. Despite his age he is an experienced player. He will add to the squad and he will bring energy and legs in midfield when we need it.'
Bradley, known as a goalscoring midfielder, said he is eager to test himself at the highest level, and can do that with Villa.
Eye for goal: Bradley impressed at the World Cup finals with the United States
'I am really excited to be joining Villa,' he said. 'Everyone can see that Villa is a big club, a fantastic club with tradition and great supporters.
'Over the past few years, Villa have made great strides and are trying to challenge at the very top of the Premier League. I am excited to be part of that and I am excited to be here.
'I want to play at the highest level and test myself in the biggest of games. Whether it's on Saturday against Fulham at Villa Park or the week after, I am excited to be here and ready to get going.'
Bradley scored 16 goals in 36 games for Heerenveen in his final season before joining Gladbach, where he managed 10 strikes in 76 appearances.
'I try to be a guy in the centre of midfield who imposes himself on the game and can get forward and get goals,' he added. 'I try to win tackles and, defensively, make it really hard on the team in the centre of midfield.'
Bradley is the son of United States head coach Bob Bradley and has won 50 caps for his country. Bradley senior was linked with Villa as a possible replacement for Martin O'Neill when he quit the midlands club in August.
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