The Danish international has played 237 games in the Premier League for both Tottenham Hotspur and, most recently, Brentford.
Eriksen also has 115 caps for Denmark and scored 38 goals for his country along the way.
Upon signing with the Red Devils, he told the club's website: "Manchester United is a special club, and I cannot wait to get started. I have had the privilege of playing at Old Trafford many times but to do it in the red shirt of United will be an amazing feeling."
Speaking about his new manager, Erik ten Hag, the player added: "I have seen Erik's work at Ajax and know the level of detail and preparation that he and his staff put into every day. It is clear that he is a fantastic coach.
"Having spoken with him and learned more about his vision and the way he wants the team to play, I am even more excited for the future. I still have major ambitions in the game, there is a huge amount that I know I can achieve, and this is the perfect place to continue my journey."
Eriksen made his first breakthrough as a professional at Ajax where he made 162 appearances across four seasons at the Amsterdam-based club.
A move to Tottenham quickly followed as the Premier League outfit splashed £11million on his signature.
While he never won a trophy with Spurs, Eriksen did enjoy a memorable six-and-a-half-year stint in North London, where he made 305 appearances and scored 69 goals across all competitions.
During his time at the club, Eriksen did reach the League Cup final in 2015 and the Champions League final in 2019 - both ending in defeats.
In January 2020, the Dane completed his move to Inter Milan where he ended up staying for two seasons, racking up 60 appearances.
The player won the 2020/21 Serie A and helped the club to the final of the 2020 Europa League, only to once again suffer defeat.
On May 25, 2021, Eriksen suffered a cardiac arrest during a UEFA Euro 2020 match against Finland.
The player collapsed in the 42nd minute and received urgent medical assistance, including cardiopulmonary resuscitation and defibrillation. He was later taken off the pitch on a stretcher before being rushed to the nearest medical facility.
Even though his life was in serious jeopardy, Eriksen recovered and returned to professional football. In 2022 he was signed by Brentford and made his first appearance for the Premier League side on February 26 against Newcastle.
Eriksen made 11 league appearances for the Bees, but despite the club's interest of prolonging the deal, the midfielder left the London side in the summer following the expiry of his contract.
The Dane will now link up with Manchester United as the club embarks on a new era under manager Erik ten Hag.
Manchester United Football Director John Murtough added: "Christian has been one of the best attacking midfielders in Europe throughout his career. It is no surprise that he had so many options this summer, so we are really happy that he was convinced that this is the right club for him.
"In addition to his outstanding technique, Christian will add valuable experience and leadership skills to the squad, and we are looking forward to seeing the benefits of these qualities on the pitch this coming season and beyond."