Marie Yovanovitch, the former US Ambassador to Ukraine had refused to hang Donald Trump's portrait in the US embassy in.
She was fired over "false claims" by people with "questionable motives"
Trump told Fox News' morning show former ambassador Marie Yovanovitch "didn't want to hang my picture in the embassy" in Ukraine. Reports-BBC
The president did not offer further details about the matter.
She was one of 12 witnesses to testify over the last two weeks in the impeachment inquiry, which is looking into alleged abuse of power by the president.
In a phone call to Fox and Friends on Friday morning, Trump referred to Yovanovitch as "the woman" and said he had heard "bad things about her".
"This ambassador, who everybody says was so wonderful," Trump said, "she wouldn't hang my picture in the embassy.
"She's in charge of the embassy, it took, like, a year and a half, two years."
He added that his personal lawyer, Rudy Giuliani, "didn't say good things" about her.
Giuliani has come up repeatedly in the impeachment hearings as witnesses say he pursued an irregular channel of US-Ukraine relations.
"She said bad things about me, she wouldn't defend me, and I have the right to change the ambassador," Trump added.
"The standard is you put the president of the United States' picture in the embassy," he continued. "This was not an angel, this woman, OK?"
It was not the first time the president has attacked Yovanovitch.
The Republican president lambasted her in a tweet while she was testifying last Friday - a move that Democrats argued amounted to witness intimidation.
A career diplomat, Yovanovitch was recalled as the American ambassador to Kyiv in May, two months before a phone call between Trump and Ukrainian President Volodymyr Zelensky that is at the heart of the congressional inquiry that may seek to remove the US president from office.
A rough transcript of July's call revealed that Trump urged President Zelensky to investigate unsubstantiated allegations against Democratic White House contender Joe Biden and claims of 2016 election meddling.
The president spent a significant portion of Friday's television interview discussing a conspiracy theory that it was Ukraine - not Russia - meddling in the US 2016 presidential election.
These claims were branded "a fictional narrative" by Thursday's impeachment witness, former White House intelligence official and Russia expert Fiona Hill.
Trump mentioned the "deep state" several times to Fox and Friends, and repeatedly claimed Ukraine had wanted him to lose the 2016 election.
"Don't forget - Ukraine hated me, they were after me in the election, they wanted Hillary Clinton to win," the president said. "When I won, all of a sudden they became so nice, they had to make up."
On Thursday, Hill did say that Ukrainian officials had spoken disparagingly of Trump, but only because they expected Hillary Clinton to win the White House.
She warned elected officials not to promote "politically driven falsehoods" that seek to cast doubt on Russia's alleged interference in US elections.
In his interview on Friday, Trump also defended his decision to dispatch Giuliani to Ukraine.
He said the former New York City mayor "is a great crime fighter" and "legendary figure".
"When you're dealing with a corrupt country, Rudy Giuliani's got credentials because of his reputation," the president said.
What next with the impeachment inquiry?
After sprinting through seven public hearings with 12 witnesses over five days, the House Intelligence Committee is now writing a report to submit to the Judiciary Committee, which is then expected to draft articles of impeachment.
The committee would begin writing the impeachment articles - or the charges of wrongdoing against the president - in early December.
After a vote in the Democratic-controlled House of Representatives, a trial would be held in the Republican-controlled Senate.
If Trump was convicted by a two-thirds majority, he would become the first US president to be removed from office through impeachment.
The White House and some Republicans want the trial to be limited to two weeks.