The livery features the papaya orange and blue colours that McLaren has used on its Formula 1 cars over the last two years.
The race will be Alonso's second attempt at the race, as he made his debut in 2017, when McLaren, Honda and Andretti Autosport formed a joined entry.
However, this time it is running independently, with Alonso sporting the number 66 which is a homeage to the first time McLaren won the Indy 500.
“The reveal of the #66 car and livery is an important moment for McLaren Racing’s journey to the Indianapolis 500," said McLaren CEO Zak Brown.
"It represents the beginning of the next phase of our programme, and reflects a tremendous amount of hard work by our specialist Indy 500 team at the McLaren Technology Centre. We are all looking forward to seeing the #66 out on track for the first time on April 24."
Alonso is searching for the Indy 500 win to become only the second driver to win the Triple Crown of Motorsport, having already ticked off the Monaco Grand Prix and the 24 Hours of Le Mans.
The Spaniard said: "I'm excited we are finally able to reveal to the world the #66 car which I'll be racing with McLaren at the Indy 500 in May.
"The team at the McLaren factory has worked very hard to build this car ready for our return to the iconic Brickyard and I think it looks fantastic in the 2019 McLaren Racing colours.
"My hopes for the race remain the same, to win and achieve the Triple Crown, and I'm looking forward to meeting the fantastic US fans who made me feel so welcome first time around."
Bahrain International Circuit - Winter testing