The 24-year-old has admitted that his compatriot and Belgium teammate played a major role in his decision to sign for the Rojiblancos on Monday.
The 24-year-old sealed a 7 million euro move to the Spanish capital from Ajax on Monday, signing a four-year deal with the Rojiblancos.
Alderweireld says that his Belgium teammate - who is on loan from Chelsea - sold the idea of life with Atletico to him.
The center back told the club's official website: "I said Atletico were interested in me and he [Courtois] told me how great it is, this team and its fans.
"He advised me to come because he is very happy here."
The Belgian, who made 128 Eredivisie appearances for Ajax, could be handed his debut by coach Diego Simeone in the Liga meeting with Almeria on Sept. 13.