Conor McGregor has caused controversy by calling Russian president Vladimir Putin "one of the greatest leaders of our time".
The MMA fighter posted a photo to Instagram of himself with Mr Putin at the World Cup final in Moscow.
Some fans weren't happy with the 30-year-old's choice to accept the Russian leader's invitation to the match in Moscow.
Mr Putin has previously been accused of human rights abuses.
But Russia has been praised for hosting a safe and enjoyable World Cup.
Today I was invited to the World Cup final as a guest of Russian President Vladimir Putin. This man is one of the greatest leaders of our time and I was honored to attend such a landmark event alongside him. Today was an honor for me Mr. Putin. Thank you and congratulations on an amazing World Cup. Россия вперёд!
According to his Instagram post, Conor was invited as a guest of Mr Putin.
The Irishman ended the post with "go Russia!" written in Russian.
Comments on McGregor's Instagram and Twitter profiles told him to "research the history".
Love you Conor but research the history— Jen Dotson (@sourmixedwkush) July 16, 2018
And there goes my support for you. A great fighter, a poor judge of character.— Stevie Williams (@stevie1666) July 16, 2018
Criticisms of Mr Putin include his policy of banning "gay propaganda", which the European Court of Human Rights has ruled is discriminatory and encourages homophobia.
The country is also accused by the UK of poisoning former spy Sergei Skripal and his daughter Yulia on British soil.
The UK didn't send any government ministers or members of the Royal Family to the World Cup as a result of the poisoning.
But Russia's World Cup has been praised as one of the best ever - both for what happened on the pitch and off it.
Many expected trouble with fans - especially after there was violence between Russian and English fans at Euro 2016 - which never materialised.
However, it has been argued that democracy, human rights and freedom of speech will be under attack again now the World Cup has left town.
Conor McGregor's Russian UFC rival Khabib Nurmagomedov was also at the World Cup final.
The pair's appearance at the match comes as the Notorious revealed his "Russian compound is taking shape" in another Instagram post - with many fans hoping it means the two stars are finally set to fight in the cage.
Newsbeat has contacted representatives of Conor McGregor for comment.