"THERE WILL BE NO THIRD DEBATE!" the former president wrote on social media site Truth Social.
Trump had participated in a debate against President Joe Biden in June before his debate against Harris on Tuesday.
Although Trump touted his performance on Tuesday against Harris, six Republican donors and three Trump advisers who spoke to Reuters earlier this week said they thought Harris had won the debate largely because Trump was unable to stay on message.
The debate attracted 67.1 million television viewers, according to Nielsen data.
Harris, speaking at a rally shortly after Trump's post went live, said: "I believe we owe it to the voters to have another debate."
While Trump said in his post that polls showed he won the debate, several surveys showed that respondents thought Harris did better.
Among voters who said they had heard at least something about Tuesday's debate, 53% said Harris won and 24% said Trump won, according to a Reuters/Ipsos poll released on Thursday.
The poll showed that 54% of registered voters believed that the single debate between Trump and Harris was enough, while 46% had wanted a second debate.
A majority of debate watchers said Harris outperformed Trump, according to a CNN flash poll released shortly after the debate. YouGov showed 54% of those surveyed said Harris won while 31% said Trump was the victor.
