Trump said he would do so to protect the future of the presidency.
"I think you have to for the sake of the office," he said Friday in an interview with Fox News.
"Especially a national security adviser. You can't have him explaining all of your statements about national security concerning Russia, China and North Korea -- everything. We just can't do that."
Bolton said Monday he'd be willing to testify if subpoenaed for the Senate's as-yet unscheduled impeachment trial.
As a key adviser to Trump, Bolton was close to the president's dealings with Ukraine. The U.S. House of Representatives last month convicted Trump on two articles of impeachment stemming from an effort to pressure the Eastern European nation into launching a politically motivated investigation of Democratic Upresidential candidate Joe Biden.
Bolton said during the impeachment hearings he would not testify before the House intelligence committee unless he was first subpoenaed by the committee and then ordered by a judge to defy Trump's wishes by appearing before Congress.
"The House has concluded its constitutional responsibility by adopting articles of impeachment related to the Ukraine matter," Bolton said. "It now falls to the Senate to fulfill its constitutional obligation to try impeachments, and it does not appear possible that a final judicial resolution of the still-unanswered constitutional questions can be obtained before the Senate acts.
"Accordingly, since my testimony is once again at issue, I have had to resolve the serious competing issues as best I could, based on careful consideration and study. I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify."
Trump told Fox News that he'd welcome testimony from acting White House chief of staff Mick Mulvaney, Secretary of State Mike Pompeo and former Energy Secretary Rick Perry -- but that doing so could also endanger the presidency.
"I want everybody -- but there are things that you can't do from the standpoint of executive privilege," the president said.
Though impeachment proceedings in the Senate have been in limbo because the House has yet to send two articles of impeachment to the chamber, House Speaker Nancy Pelosi announced that could change next week. She said she instructed the judiciary committee to prepare to bring the articles to the floor next week for a resolution to transmit them to the Senate.