Cristiano Ronaldo in his early days with Manchester United
The teenager was snapped up for £12.24 million, making him the most expensive teenager in the world - a record that didn't last long with United alone having snapped up Anderson (£22 million), Wayne Rooney (£26 million), Luke Shaw (£31 million) and Anthony Martial (£36 million) in recent years.
Ferguson had been in pursuit of Ronaldo for a while, but had to accelerate proceedings as other sides became interested in the Portuguese teenager.
Upon signing the youngster, Ferguson said: 'He is an extremely talented footballer, a two-footed attacker who can play anywhere up front: right, left or through the middle.'
It took time for Ronaldo to scale new heights for his club
Ronaldo was assigned the number 7 jersey vacated by David Beckham that summer, but initially struggled to live up to the hype that surrounded him.
He was often lambasted for not passing to his teammates and showboating. His role in getting Wayne Rooney sent off during Euro 2004 - an incriminating wink to the Portugal bench wasn't particularly well disguised - didn't help ingratiate himself to the United faithful, though the incident proved not to have too damaging an effect.
Though Ronaldo is now renowned for his tremendous feats of goalscoring, he was not always quite so prolific.
In his first four seasons at the club, he scored 50 goals for United. But in the 2007/8 season, Ronaldo reached new heights, scoring 42 goals in all competitions and helping United lift the Premier League and Champions League trophies.
Despite rumours that he would leave for Real Madrid, he remained at Old Trafford and scored 25 goals the following season, winning the Premier League for one final time but losing in the Champions League final to Barcelona.
Ronaldo was finally granted his wish during the summer of 2009 as he signed for Real Madrid in a world-record £80 million deal.
Ronaldo is unveiled as a Galactico
His goalscoring feats at the Bernabeu have been scarcely believable. He has scored 351 goals in 304 appearances for the club, winning the Champions League twice and the Ballon d'Or on three occasions.
This summer, he finally achieved success on the international front as his Portugal side - shorn of their talisman through injury minutes into the final - defeated the hosts France 1-0 to win Euro 2016.
Since that fateful day 13 years ago, Ronaldo has evolved to become one of the world's most complete players, vying with Lionel Messi year-on-year to be crowned the world's best player.
For the player revealed alongside Ronaldo on 12 August 2003, things didn't work out quite the same.
The other Ronaldo may have scored the goals, but Brazil's 2002 World Cup win was attributed elsewhere
Ferguson announced not only the signing of Cristiano Ronaldo but also of world cup-winning midfielder, Kléberson.
The 24-year-old Brazilian was signed from Atlético Paranaense for £6 million, having been given a ringing endorsement from Brazil manager Luiz Felipe Scolari.
Kleberson - Manchester United
Scolari had said that Kléberson 'was always the first name on the team list, ahead of players like Ronaldo.'
He could only muster 20 league appearances during his two seasons at the club, before being sold to Beşiktaş for £2.5 million.
The 37-year-old Kléberson can now be found playing in America for the Fort Lauderdale Strikers.