Portland does not have a sheriff but the Multnomah County sheriff represents the city's area in Oregon.
Mr Trump made the claim during the first US presidential debate on Tuesday, as he tried to boast that "almost every law enforcement" supports him.
In tonight’s presidential debate the President said the “Portland Sheriff” supports him. As the Multnomah County Sheriff I have never supported Donald Trump and will never support him.— Mike Reese (@SheriffReese)September 30, 2020
At one point, Mr Trump said to Mr Biden: "Almost every law enforcement group in the United States — I have Florida, I have Texas, I have Ohio, I have everyone — excuse me, Portland sheriff just came out today and said 'I support President Trump'.
"I don't think you have any law enforcement."
However, Mr Reese quickly slammed Mr Trump's claim, tweeting: "In tonight's presidential debate the President said the 'Portland Sheriff' supports him.
"As the Multnomah County Sheriff I have never supported Donald Trump and will never support him."
Mr Trump also said to Mr Biden: "You can't even say the word law enforcement because if you say those words, you're going to lose all of your radical left supporters."
The president frequently discussed: "Law and order" having tweeted those words on at least four different occasions over the last few months.
It came as protests occurred around the country against racial injustice and police brutality, some of which turned violent.
During the debate, Mr Trump slammed cities run by Democrats that he claims experienced more violence in their demonstrations.
Portland has a Democratic Mayor, Ted Wheeler and Oregon has a Democratic Governor, Kate Brown.