The #66 McLaren entry will be piloted by Alonso in what will be his second attempt on the Indy 500, with victory enough to seal the triple crown of motorsport.
It has been designated as #66 in tribute to McLaren first win at the Brickyard with Mark Donohue aboard the Penske-run M16 in 1972.
The McLaren Indycar has been built at the McLaren Technology Centre under the banner of McLaren Racing and using what McLaren is calling its "specialist" staff. The McLaren entry will be supported by Carlin Racing for the Indy 500.
Alonso's #66 car is currently travelling to the US ahead of the open test at the Indianapolis Motor Speedway on April 24.
A car McLaren is designating as its test machine will be sent to Texas Motor Speedway so that Alonso can drive a universal aerokit-equipped Indycar on an oval for the first time on April 9.
Like its F1 car, the McLaren Indycar is decked in Papaya orange, which McLaren says "reflects the livery of the McLaren F1 team's MCL34" while both cars feature a "complementary blue" that nods to its heritage.
Alonso 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."
McLaren Racing CEO Zak Brown added: "The reveal of the #66 car and livery is an important moment for McLaren Racing's journey to the Indianapolis 500.
"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."