The 29-year-old, who wears the number 18 shirt vacated by Casemiro, has long been linked with United going back to his breakthrough as a 16-year-old at Anderlecht in 2013. His career since has taken him through Monaco, Leicester City and Aston Villa, winning the FA Cup with the Foxes and the Europa League with Villa along the way.
"I think it's a great step up for myself. The club is in a great place, and I feel like there is a lot of ambition inside the club," he said in his signing interview.
"Inside the team, there's a lot of quality and I feel like this is a good step for me to push on and hopefully win many, many titles with the club.
"I'm very happy and very excited to start, and to meet my team-mates and be on the pitch together."
Tielemans brings leadership experience with him, having worn the armband for Anderlecht in Europe as a teenager and been chosen ahead of Kevin De Bruyne and Thibaut Courtois to captain Belgium in North America this summer.
"I have big experience, 29 years old and I'm ready to push on and make the next step in my career," he added. "That's why this is the perfect club for me.
"They want to win and, hopefully, be really good on the pitch, so that's why I chose to come here.
"Being in that leadership role has been really good for me. I find myself naturally in that and, hopefully, I can bring that into the team."
Part of the appeal, too, is working under Michael Carrick, whose second half of last season saw United post the best run of form of any Premier League side in 2026 - form good enough to finish one place above Aston Villa, despite Tielemans' side having led the Reds by 10 points at Christmas.
"I'm looking forward to it," he said.
"Obviously, him being a midfielder, he can give me a lot of tips and I can learn from him inside his structure.
"I'm looking forward to learning from him and linking up with my team-mates."
The pair's history goes back further than most realise. Tielemans faced Carrick as a teenager when Anderlecht met United in the 2017 Europa League quarter-finals, a tie that finished 1-1 in Brussels before United won the return leg through a Marcus Rashford extra-time goal at Old Trafford.
"I saw the picture [shaking hands with Carrick] and it was actually really funny, when we talked to each other. It was good times and, hopefully, better times now.
"Yeah, it was a great memory, Manchester United is a massive club and to play at Old Trafford for the first time, it was big. I'd never seen a stadium that big in my life before, so it was amazing.
"Once you're on the pitches, it's on to football and what you can do and what you've done your whole life, so I was very excited to play that game and now, obviously, very excited to be in those colours.
"You can feel the atmosphere and history, so to play for the home team it's going to be nice."