The Bronze Bomber took to Twitter just minutes after AJ was beaten by Andy Ruiz Jr in New York.
Joshua, 29, was dropped four times before the referee brought the bout to an end in the seventh round, taking the WBA, IBF, IBO and WBO titles away from the Watford man.
And WBC champ Wilder, 33, claimed AJ was “not a true champion”, brutally mocking his rival.
He tweeted: “He wasn’t a true champion.
“His whole career was consisted of lies, contradictions and gifts.
“Facts and now we know who was running from who!!!! #TilThisDay”
Joshua’s promoter Eddie Hearn was read the tweet during his post-fight press conference and hammered Wilder for lacking class.
Hearn said: “I’m sure there will be lots of people who are delighted AJ lost tonight.
“Deontay Wilder has zero class for kicking Joshua while he is down.”
Wilder knocked out Dominic Breazeale in the first round of their bout last month to retain his WBC title for the ninth time.
Like Joshua taking on Ruiz, Wilder was seen as taking the easy option by fighting Breazeale.
But while the American savagely dispatched his opponent, AJ simply could not do the same.
Despite somehow being UP on one of the judge’s scorecards, he was dominated by the Mexican-American for the majority of the bout and was deservedly beaten.
Joshua was accused of “giving up” and “lacking chin” by some fans but promised to return after the major shock to the 25-1 underdog.
He said: “I got beaten by a good fighter.
“It will be interesting to see how far he goes, but this is all part of the journey.
“He’s a champion for now, I shall return.”
Joshua is now expected to take a rematch with Ruiz towards the end of this year to reclaim his belts.
Wilder, meanwhile, will fight Luis Ortiz next and has then promised that an agreement has been reached for a Tyson Fury rematch. Wilder vs Ortiz Boxing