The 22-year-old has thrived at Abafana Bes'thende since his promotion to the first team in the 2023/24 season.
He was reportely a target for a number of top-flight teams, and was even said to be close to joining Betway Premiership Champions Orlando Pirates - but the Brazilians have won the race for his signature.
In total, the talented Ndlovu made 75 appearances, scoring eight goals and providing 13 assists for Arrows.
A Sundowns post read: "Siyanda Ndlovu has arrived! Welcome to the home of Champions. Masandawana, let's welcome him."
Ndlovu is Sundowns' second signing of the off-season after the arrival of Antonio Van Wyk from Austrian side SV Ried.
Siwelele FC have confirmed the departure of goalkeeper Ricardo Goss following the conclusion of his loan spell, bringing an end to a successful season in Bloemfontein as the club continues reshaping its squad ahead of the new campaign.
The South African Police Service are investigating a case of attempted murder after Mamelodi Sundowns defender Divine Lunga escaped a shooting incident in Johannesburg.
Mamelodi Sundowns versatile wingback Thapelo Morena has set his sights on having an injury-free 2026/27 season and contributing more to the team's success.
South African forward Cassius Mailula is a free agent after parting ways with Major League Soccer club Toronto FC during the current season.
