The comedian and former Britain's Got Talent judge posted a string of snaps on Instagram showing Alfred - who he shares with his ex Lara Stone - as Spaceboy, from his latest children's book.
Alfred was holding his dad's hand as he walked to school in a space suit, with his face covered by a shiny silver helmet.
"Me & my little SPACEBOY for #WORLDBOOKDAY," Walliams wrote.
Walliams was one of a host of celebs getting into the spirit of World Book Day.
Myleene Klass went all out when it came to outfitting her three-year-old son Apollo as a dinosaur, even making up his face to look like scaly skin.
Girls Aloud star Kimberley Walsh admitted that the special day took her by surprise so she wasn't fully prepared when it came to costumes for her two boys, Bobby and Cole.
Luckily, their school suggested parents recycle things from home so she was able to rustle up a secret agent outfit and a cops and robbers suit.
Sharing pictures of them on Instagram, the singer said: "Needless to say World Book Day crept up on us again but thankfully the school wanted everyone to be sustainable and work with what we had at home so that’s what we did.
"Bobby rocking his fave suit as the Accidental Secret Agent by Tom McLaughlin (with an actual spy camera pen he already had) and Cole the old Cops and Robbers by Janet & Allan Ahlberg."
"All was well until Bobby freaked out about not having his curls seconds before he went into school," added Walsh, who is also mum to baby son Nate.
Amanda Holden didn't share snaps of what her daughters dressed up as but she did mark the day herself.
The TV star dressed up as Dorothy as she and her Heart colleagues Jamie Theakston and Ashley Roberts channelled L Frank Baum's The Wizard Of Oz.
The star shared a video on social media that showed her in a short blue gingham dress and a white blouse.
Dancing around with her hair in bunches, she was arm in arm with Roberts, who was dressed as the lion, and Theakston, who was the tin man.
Watch: Queen Consort talks reading with Children's Laureate for World Book Day