The character first seen in 2021's The Suicide Squad is described as a brazen hero who uses violence to achieve peace at any cost.
Cena's tweet quickly went viral with fans and critics calling out his "insensitive" words.
One fan wrote, "I'm just saying there is never an appropriate moment to say this, especially now."
"Great time to hashtag your show for trending purposes … very insensitive don't you think?" another asked.
Another suggested that the actor might instead use his own wealth to help those struggling.
"I hope you didn't mean to come off as disrespectful by using this to promote your show," they wrote.
"You don't have the powers of peacemaker but you do have the power of money and status. Please donate to help Ukraine or spread the link to donate."
"A bit tone deaf @JohnCena. Please delete. [This is from] a conversation I'm in: Families have literally been blown to pieces in Ukraine. Meanwhile Cena, 'I AM A COMIC BOOK ACTOR, IF MY CHARACTER WAS REAL, I WOULD STOP IT FROM HAPPENING,' " a fan explained.
"I feel like I shouldn't have to explain this to you but the power of peacemaker is murdering people," someone else then pointed out.
This is not the first time Cena has found himself under fire over international affairs. In 2021, he was hit with fierce criticism for kowtowing to the Chinese Communist Party while on a promotional tour for the Fast and Furious franchise. He apologised to China at the time for calling Taiwan a country and said the apology in Mandarin.
"Hi China, I'm John Cena. I'm in the middle of Fast and Furious 9 promotions. I'm doing a lot of interviews. I made a mistake in one of my interviews," he said.
"I made one mistake. I have to say something very, very, very important now. I love and respect China and Chinese people. I'm very, very sorry about my mistake. I apologise, I apologise, I'm very sorry. You must understand that I really love, really respect China and the Chinese people. My apologies. See you," he continued.
However, the actor is also known for his incredible acts of generosity to fans. Cena is prolific when it comes to granting wishes for the Make-A-Wish Foundation. Overall, he has granted over 650 wishes, making him the leading celebrity to gift the most wishes for the foundation.