The puzzle books starring the red and white jumper-wearing traveller first launched in 1987, and have gone on to sell over 58 million copies worldwide.
The London Eye was lit up in red and white last night to mark the milestone, while a special birthday message was beamed across the city skyline.
A message and picture of the character will also be seen on Piccadilly Circus screens today (September 21).
The book series has been printed in 30 languages and sold in 38 countries worldwide, and is known as Where's Waldo? in the US.
Artist Martin Hanford originally placed the character into one of his crowd pictures to see if people could find him.
Where's Wally? has also seen TV show, comic strip and video game spinoffs.
3,701 people helped break the Guinness World Record for the most 'Wally's in one place at the Street Performance World Championships in Dublin in 2011.