Marie Yovanovitch was removed from her post as a senior diplomat by the Trump administration earlier this year. Ms Yovanovitch defended her anti-corruption record during the hearing. She claimed her removal had left US policy there in disarray.
As she testified, Trump bombarded Twitter with criticisms of her.
He said: “Everywhere Marie Yovanovitch went turned bad.
“She started off in Somalia, how did that go?”
The Democratic chairman of the House Intelligence Committee Adam Schiff then asked Ms Yovanovitch for her reaction to the tweet.
She replied she found it “very intimidating”.
Ms Yovanovitch added: “I can’t speak to what the president is trying to do, but I think the effect is to be intimidating.”
Mr Schiff replied: “Well, I want to let you know, ambassador, that some of us here take witness intimidation very, very seriously.”
House Speaker Nancy Pelosi was asked if the tweet was classes as witness intimidation.
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“Come on. Why would we start making that judgement now?”
During a call with the Ukrainian President Volodymyr Zelensky, Trump described Ms Yovanovitch as “bad news”.
A whistleblower’s report of the call sparked the impeachment inquiry.
Ms Yovanovitch described the moment she read the memo of the conversation.
She said: “I was shocked and devastated that I would feature in a phone call between two heads of state in such a manner, where President Trump said that I was ‘bad news’ to another head of state.
“A person who saw me reading the transcript said that the colour drained from my face, I think I even had a physical reaction.
“Even now words kind of fail me.”
The impeachment inquiry is over Trump allegedly absuing the power of his office by pressing Mr Zelensky to investigate a political rival, former Vice President Joe Biden.