In what was arguably one of the stranger moves of transfer deadline day, the Blues forked out £34m to bring the 29-year-old defender back to Stamford Bridge.
The Brazil international returns to England following a two-year stint in Paris, where he made 89 appearances in all competitions for Paris Saint-Germain.
"I think David is a very good player," Courtois told reporters. "It is an interesting move and I hope he will help us win titles. It's good to have him with us."
While the signing of Luiz will no doubt be at the forefront of fans' thoughts, the news that Diego Costa is staying at Stamford Bridge is also a massive boost for the club.
The 27-year-old recently admitted he had considered a return to former employers Atletico Madrid after a difficult year in England, which ultimately cost him a place in Spain's Euro 2106 squad.
Courtois added: "I believe that he has had two good seasons at Chelsea. I have not talked to him really about the rumours over a return to Atletico Madrid.
"I am pleased he is staying at Chelsea because he is a great forward and he has already scored two really important goals for us this season.
"We were talking at Chelsea. Diego was disappointed not to go to the Euros and now he has the chance to show his quality as the forward of the national team."