Wladimir and Vitali Klitschko have launched a joint appeal in wake of the tension between Ukraine and Russia.
Russia has launched an invasion of Ukraine after president Vladimir Putin ordered a 'specialised military operation'.
Last month, as the relationship between the two countries intensified, Wladimir enlisted in the Ukrainian reserve army.
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He issued a statement on social media after reports of missile strikes that targetted airports and military bases.
"The world is watching how reckless and deadly imperialism is, not just for Ukraine but the whole world," he wrote on Twitter.
"Let history be a lesson to not be repeated."
Wladimir then joined his older brother Vital, who's the major of Kiev, to post an appeal as a result of the invasion of Ukraine by Russia.
In the video, he says: "I’m calling to all international partners to observe this tragedy that is happening nowadays in Ukraine.
"And this senseless war that is not going to have any winners, but losers.
"I just want to tell you we must stay united against this aggression, against Russian aggression.
"Don’t let it continue happening in Ukraine, don’t let it happen in Europe and eventually in the world.
"United we are strong, support Ukraine, thank you."
The football world has also reacted to the full-scale invasion of Ukraine, with Manchester City defender Oleksandr Zinchenko slamming Putin's decision to launch an assault.
The Ukraine international posted a picture of Putin on his Instagram story, alongside the caption: "I hope you die the most painful suffering death, creature."
Dynamo Moscow striker Fedor Smolov then became the first high-profile Russian footballer to speak out against the war.
He posted a black square with the caption 'No War,' which included a Ukraine flag and a broken heart emoji.
Topics: Wladimir Klitschko, Boxing
Nasir Jabbar