Users of the online multiplayer game complained their profiles were hacked with the message: “Ask your parents to vote for Trump this year!” and “MAGA2020”, an abbreviation of Make America Great Again.
Roblox has more than 100 million users worldwide.
It is marketed at children aged between nine and 14.
Players have reported that in-game messages have been sent from the hacked profiles to the account holders' friends, urging them to support Trump in the November vote .
Hacked avatars have been dressed in existing in-game accessories which resemble items worn by Trump supporters.
This includes a red hat with “Running of the bulls” written in white, which looks like the Make America Great Again hats that the US president favours.
In addition, some characters are altered to wear a T-shirt featuring an American Eagle.At the time of this writing, there were more than 1,000 profiles in Roblox that featured the phrase: "Ask your parents to vote for Trump this year!"
Some players have told the news site Bleeping Computer that they are still able to access their accounts and can change their profiles back again.
“A Trump supporter hacked my Roblox account and messaged my friends to vote for him,” one gamer wrote on Twitter.
“Why is my avatar this?” another wrote with a photo of their avatar. "I’m not even American."
Roblox did not immediately respond to the BBC’s request for comment.