The Brazil international has penned a two-year deal at Arsenal after quitting Chelsea.
Arsenal coughed up £8m for Luiz, 32, who was desperate to head across London to the Emirates.
Luiz will wear the No. 23 shirt at the Emirates this season.
Unai Emery is a big fan of Luiz and the Arsenal boss hailed his new signing.
“David has huge experience and I look forward to working with him again,” he said.
“He is a well known player and adds to our defensive strength.”
Luiz didn’t train with Chelsea on Wednesday after a fallout with club staff.
Frank Lampard deemed the Brazilian surplus to requirements at Stamford Bridge and ultimately allowed him to join Arsenal.
Luiz is Arsenal’s fifth summer signing, following Kieran Tierney, Nicolas Pepe, William Saliba, Dani Ceballos and Gabriel Martinelli to the Emirates.
However, Saliba has been loaned straight back to Saint-Etienne.
Luiz and Tierney both joined on deadline day before the 5pm cut-off.
And £25m signing Tierney revealed it was a hard decision to quit his boyhood club Celtic for Arsenal.
He said: “Yeah, it was the hardest decision in my whole life that I’ve had to make.
“I had to speak to my family and my close friends and my agent as well because it’s a hard decision to leave a club that you love so much.
“People need to know that as well.
“I feel Arsenal is a massive club and it’s a great opportunity for me as well.
“I’m delighted to be here.”