Joshua has been a long-time rival of Whyte from back in their amateur days but he has lent his fellow heavyweight his support this evening.
Whyte and Parker are going head-to-head at London’s O2 with the winner likely securing a shot at the world champion.
Both fighters have only lost once prior to their showdown – and both of those defeats came against Joshua.
And the unified world champion has taken to Twitter to give his backing to his fellow Brit.
“Even though we’re rivals I wish you the best of luck tonight Dillian,” Joshua wrote.
“Tune in tonight to support British boxing. Loving it.”
Joshua is currently in training for his title defence against Alexander Povetkin on September 22.
But he has previously tipped Whyte to do what he could not earlier this year and knockout Parker.
“It’s a very, very good fight. If you ask me who I’m backing, I’m going Dillian – and I’m going by knockout as well,” he told Sky Sports.
“Dillian hasn’t got anything to lose, he’s been screaming out world champions saying ‘I want to fight here and I want to fight there’ and I feel like he wants to prove himself.
“Parker is strong, he can take a shot but I feel like in my fight, he was on the back foot.
“I boxed clever because I’m trying to set up the big fight with [Deontay] Wilder so I wanted to box clever to secure the next one without taking any risks.
“But I feel like Dillian hasn’t got anything to lose and everything to gain so he’ll go in there all guns blazing and try to knock out Parker ASAP so he can secure the win and get that world title shot.”