In a letter to House Democrats, Ms Pelosi said: “This morning, I spoke to the Chairman of the Joint Chiefs of Staff Mark Milley to discuss available precautions for preventing an unstable president from initiating military hostilities or accessing the launch codes and ordering a nuclear strike.
“The situation of this unhinged President could not be more dangerous, and we must do everything that we can to protect the American people from his unbalanced assault on our country and our democracy.”
The letter came just before Ms Pelosi was to join a telephone conference to discuss their options for impeaching the outgoing President.
Earlier this week, violent and deadly riots broke out in Capitol Hill, carried out by Trump supporters.
A total of five people have been killed so far after Mr Trump protested against his election defeat and urged supporters to “fight” to overturn the results.
Ms Pelosi said: “As you know, there is growing momentum around the invocation of the 25th Amendment, which would allow the Vice President and a majority of the Cabinet to remove the President for his incitement of insurrection and the danger he still poses.
“Yesterday, Leader Schumer and I placed a call with Vice President Pence, and we still hope to hear from him as soon as possible with a positive answer as to whether he and the Cabinet will honour their oath to the Constitution and the American people.
“Nearly fifty years ago, after years of enabling their rogue President, Republicans in Congress finally told President Nixon that it was time to go.
“Today, following the President’s dangerous and seditious acts, Republicans in Congress need to follow that example and call on Trump to depart his office – immediately.
“If the President does not leave office imminently and willingly, the Congress will proceed with our action.”
President-elect Joe Biden is set to be inaugurated on January 20, which Mr Trump has already refused to attend.
But fears the Republican could declare war were previously raised following his brutal election defeat.