Mr Napolitano, a former New Jersey superior court judge whose views have previously been parroted by the US president, said Mr Trump's obstruction of the House impeachment inquiry was alone enough to put him on trial in the Senate.
“I believe that the Democrats have credibly argued that he committed impeachable offences.” Mr Napolitano told Fox News show America’s Newsroom.
“The easiest one ... is obstruction of Congress. So by directing his subordinates to refuse to comply with lawfully issued subpoenas, whether it’s for testimony or for documents, that’s an impeachable offence.”
Asked by host Bill Hemmer if he would vote to impeach Mr Trump if he were a congressman, Mr Napolitano responded he “certainly would” over obstruction, adding “reasonable minds” could differ on the charge Mr Trump attempted to pressure Ukraine into investigating his political rivals.
“What was the president’s intent when he held up this aid? Was it truly to eradicate corruption in Ukraine?” Mr Napolitano said, adding: “Or was he looking to use the instruments of a foreign government to help his political campaign? Reasonable minds can differ on that.”
Mr Napolitano’s comments came before House speaker Nancy Pelosi announced on Thursday Democrats would take a crucial step towards a vote in the House by drafting articles of impeachment against Mr Trump.
The articles will likely encompass Democrats' findings on Mr Trump's dealings with Ukraine. Democrats are still writing them, but the articles could charge Mr Trump with abuse of office, bribery and obstruction.
Lawmakers and staff are expected to finish drafting the articles in the coming days, a process that is being led by Ms Pelosi and the judiciary committee. That panel could approve the articles as early as next week, setting up a vote of the full House in the days before Christmas.
If the vote to impeach the president passes, Mr Trump would then face a trial in the Senate.
Additional reporting by AP