Former U.S. Open champion Cilic, who beat Novak Djokovic for the title last year, will once again use the tournament to fine-tune his preparation for Wimbledon.
"Winning the (title) last year was one of the highlights of my career," Cilic, who also triumphed in 2012, said in a statement.
"It was a close final and fightback against Novak and came after I was narrowly beaten by Feliciano Lopez in the final a year earlier.
"I have always loved playing at the Queen's Club, and it will be a special feeling to try to defend my title in 2019."
Tsitsipas, who stunned 20-times Grand Slam champion Roger Federer at last month's Australian Open, will make his debut in the grass court tournament.
"I've heard so much about this club, so many great champions have lifted the trophy over more than 100 years, and I am looking forward to having my own chance to compete on grass for the title in June," last week's Marseille Open winner Tsitsipas said.
(Writing by Shrivathsa Sridhar in Bengaluru; Editing by Ossian Shine)
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