The Arsenal forward was left out of Roy Hodgson's 26-man party.
Hodgson opted against the Arsenal forward's inclusion in his 26-man party for Euro 2016, picking instead Newcastle United winger Andros Townsend and Manchester United's teenage striker Marcus Rashford.
The decision means that Walcott will not feature at an international tournament for the second time running, having been ruled out of the 2014 World Cup through injury.
The 27-year-old posted: "I am of course disappointed not to make the squad, but I have spoken with Roy and respect his decision.
"I wish Roy and all the team the best of luck for a successful Euros."
I am of course disappointed not to make the squad, but I have spoken with Roy and respect his decision— Theo Walcott (@theowalcott) 16 May 2016
I wish Roy and all the team the best of luck for a successful Euro's.— Theo Walcott (@theowalcott) 16 May 2016
Hodgson defended Walcott's omission by claiming he hadn't played regularly enough throughout the campaign.
However, Walcott's Arsenal teammate Jack Wilshere was included despite making 30 fewer league appearances - starting just once.
The former Southampton prospect, who has 43 caps for the Three Lions, was linked with a £20 million move to West Ham United last week.
Everton defender Phil Jagielka and Sunderland striker Jermain Defoe, who scored 15 league goals to keep the Mackems in the Premier League, were other notable omissions from Hodgson's squad, which will be cut to 23 ahead of the tournament.
Gunners pair Danny Welbeck and Alex Oxlade-Chamberlain miss out through injury, while Kieran Gibbs has not been called up.