The Crystal Palace youngster was tipped to replace his rival from Tottenham Hotspur.
Trippier has spent the best part of two years as Englabd's first-choice at right wing-back, and starred at last summer's World Cup in Russia.
But the 28-year-old has struggled for Tottenham this season and was widely tipped to lose his place to Wan-Bissaka, who in contrast has shone for Palace.
Instead, it was Trippier who made the cut, alongside his former Spurs teammate Kyle Walker and Liverpool's Trent Alexander-Arnold in Southgate's 25-man selection.
And several of a Three Lions persuasion - including those who don't support Tottenham or Crystal Palace - are voicing their opinions on social media.
No Wan Bissaka but Trippier?— Jack.A (@thfcjack_) 13 March 2019
How is Trippier been picked over Aaron Wan Bissaka lol— Lᴇᴡɪs (@RubenReflex) 13 March 2019
Trippier and no wan Bissaka— craigy (@craig_barrick96) 13 March 2019
That England squad is quite funny how’ve you got trippier in there but not wan Bissaka looool— Brogan (@blrue8_) 13 March 2019
trippier made it but not aaron wan bissaka, what? https://t.co/Nqpa9MUxpb— uncle khalid (@xxxporneditor) 13 March 2019
Kieran Trippier in the England squad instead of Wan Bissaka. Well done Gareth you muppet— PL Polls (@PL_PoIIs) 13 March 2019
Trippier over Bissaka? Southgate must struggle to pick his underwear in the morning— John Cavanagh (@cavz9) 13 March 2019
Not even Spurs fans rate Trippier right now, Wan Bissaka is the clear choice.— Dan Critchlow (@afcDW) 13 March 2019
I hate the way national team managers play favourites instead of just picking their best players. https://t.co/gRYWNeDKfb
Wan Bissaka made his debut for England's Under-21s late last year but is still awaiting his first senior cap.
The Crystal Palace academy product has been linked with a £40 million summer move to clubs including Bayern Munich, Borussia Dortmund and Tottenham's top-four rivals, Chelsea, Manchester United and Manchester City.