Star US decathlete Ashton Eaton has come under fire from Olympic viewers after wearing a Canada hat while cheering on his wife, Canadian heptathlete Brianne Theisen-Eaton.

Defending Olympic champion Eaton was seen sporting a red "Canada" baseball cap at the Olympic track meet on Saturday as Theisen-Eaton began her quest for a medal.

He's since come under fire on social media, labelled a traitor, accused of breaking United States supporters' hearts and criticised for not finding a way of supporting his wife other than adopting another country's colours.

Eaton was forced to hit back via his own Twitter account.

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"Have I not represented USA well? Yet u demand more. Ur respect is hard earned. I support the country that produced my wife; who ru2 shame me?" he tweeted.

"Are your grandparents from another country? Will your grandchildren be? What then? Hopefully you'll adopt a more reasonable view."

Many users also jumped to his defence, applauding him for being a supportive husband.

Athletics Canada then joined the conversation, backing Eaton, in true Olympic spirit.

"We appreciate the support for your wife, & we're cool with Bri supporting (the US) when you compete," the organisation tweeted.