The 26-year-old Croatian is sixth in the standings and now guaranteed a spot at the prestigious tournament from Nov. 9-16 courtesy of his grand slam victory at Flushing Meadows.
One place in the tour finals is reserved for a major winner who is outside of the top seven in the race standings.
The only scenario that could have led to Cilic missing out would have been if he dropped outside the top eight and Australian Open champion Stan Wawrinka ended up eighth, which now cannot happen.
Joining him in London will be Novak Djokovic, Rafa Nadal, Roger Federer and Wawrinka. Kei Nishikori, Tomas Berdych and Milos Raonic currently occupy the remaining three places.
Cilic will be the first Croatian to play at the season finale since Ivan Ljubicic qualified for the 2006 Tennis Masters Cup in Shanghai.
"This was one of my childhood dreams," said Cilic. "The ATP World Tour Finals is for sure one of the most interesting tournaments of the year. It is watched all over the world and brings a big attendance to all the matches. It will be a big honour for me to participate there."
The world number eight, coached by 2001 Wimbledon champion Goran Ivanisevic, won his first grand slam at this year's U.S. Open.
He won ATP World Tour events in Zagreb and Delray Beach and finished runner-up in Rotterdam.
Cilic is also through to the Kremlin Cup semi-finals in Moscow.
(Reporting By Dmitriy Rogovitskiy, editing by Ed Osmond)
