The Spaniard will still race for his F1 employers McLaren, who are returning to Indianapolis after a 38-year absence.
However, it raises fresh speculation that this will be Alonso’s last season in F1 – even as McLaren insist he is swapping codes with their blessing.
Alonso told McLaren’s official website:
I’ve never raced an IndyCar car before, and neither have I ever driven on a super-speedway, but I’m confident that I’ll get to grips with it fast.
"Of course, [it’s] a regret of mine that I won’t be able to race at Monaco this year. But Monaco will be the only 2017 Grand Prix I’ll be missing, and I’ll be back in the cockpit of the McLaren-Honda MCL32 for the Canadian Grand Prix in Montreal in early June."
The two-time world champion is still awaiting his first podium finish since moving back to McLaren in 2015, with poor pace and reliability issues leaving him a disillusioned figure in the paddock.
He started 2017 with two disappointing finishes, both outside the points, with reports suggesting he could walk away from the sport when his contract expires at the end of the season.
Read the full article on eurosport.co.uk: Fernando Alonso to miss Monaco Grand Prix to race at Indy 500, no replacement named yet