The midfielder, 20, agreed to a five-year deal with the French champions.
He scored 13 league goals for Anderlecht last season to help them win the Belgian title.
"Monaco is a club with an ambitious project, recognised on the European stage, and which relies on young players," Tielemans said.
"I am very happy to be here. I will now do everything possible to progress here and repay the confidence the club have shown in me."
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It said the scale of footpath erosion was "threatening Snowdonia's fragile nature".
The number of walkers climbing Snowdon has doubled since 2007 and more than four million people visited Snowdonia last year.
But the trust said grass species move in when pathways are eroded, strangling some plants.
Rhys Thomas, National Trust ranger for Snowdonia, said: "I've been building and rebuilding paths in the area for eight years.
"It's tiring, time-consuming and back-breaking work - involving hundreds of hours of volunteer time which requires shifting tonnes of stone by hand, vehicle and helicopter.
"But it's vital if we want to protect the delicate upland habitats that make Snowdonia a special place for wildlife."
Plants which could be at risk include purple saxifrage, moss campion and roseroot.
It is also home to the endangered Snowdon beetle - which in the UK are only found in Snowdonia.
The trust is trying to raise Â£250,000 to repair two-and-a-half miles of path.
It comes as concerns grow over the amount of litter being left on Snowdon, with some calling for a walker's charge to be introduced.
The appeal is supported by Hollywood actor Matthew Rhys, National Trust Wales's Snowdonia Ambassador.
Mr Rhys, who is from Cardiff, but has strong family connections to the national park, said he had seen the "trail of destruction left by the eroded paths" during visits to the area.
"There's a delicate balance to be struck between man and nature," he added.
National Trust Wales looks after almost 58,000 acres of mountain and farmland across the Snowdonia National Park.

Both Real and Atletico Madrid were banned after breaching rules over the transfer of foreign players under 18.
As part of the ban, football's world governing body Fifa said the young players involved were not allowed to play.
However, Cas has agreed to suspend that requirement while the appeal is heard.

The Swans are bottom of the Premier League having won just twice all season and face Sunderland on Saturday.
"We haven't gone into individuals sort of money but they are fully committed to make some signings to give everybody a lift to go forward," he said.
"We need some players in to give everybody a lift."
American owners Jason Levien and Steve Kaplan bought a controlling stake of 68% of shares in the club in July.
Manager Bob Bradley's job could be on the line after he was made bookies' favourite to be the next Premier League manager sacked but Jenkins says the club is focused on avoiding relegation.
"We don't control every story that goes out in the press. The recent story about Bob Bradley, I don't know where that came from," he continued.
"Our main aim is to make sure we get things right over the next three games and have a few discussions over the next few weeks to make sure we strengthen the team in January.
"We are fully committed to try and make sure we compete and stay in the Premier League every year."
Jenkins has said he takes responsibility for the club's slump, which has left them bottom of the Premier League.
Fans have turned on Jenkins and the board, accusing them of greed after selling most of their shares in July's American takeover.
Monaco have signed Belgium's young player of the year Youri Tielemans from Anderlecht for a reported 25m euros (£21.63m).
[NEXT_CONCEPT]
Snowdonia's pathways need to be preserved or plants and wildlife will be lost, National Trust Wales has said.
[NEXT_CONCEPT]
Real Madrid has told the Court of Arbitration for Sport (Cas) it will appeal against its Fifa ban on signing players.
[NEXT_CONCEPT]
Swansea City chairman Huw Jenkins has agreed with the club's American owners that money will be available for transfers as they fight relegation.