Ms Pelosi was advocating on behalf of undocumented immigrants who came to the US as children, known as "Dreamers".
She wants them protected as part of a budget deal being agreed this week.
The House minority leader began her speech at 10:04 local time (15:04 GMT) - and did not finish until the evening.
Dreamers were previously protected under the Deferred Action for Childhood Arrivals scheme - something which was ended by President Donald Trump last year.
"Every day, courageous patriotic Dreamers lose their status, every day the American Dream slips further out of reach," Ms Pelosi told assembled lawmakers.
"As members of Congress, we have a moral responsibility to act now to protect Dreamers who are the pride of our nation and are American in every way but on paper."
She then shared dozens of personal stories from the immigrants facing deportation, effectively halting business in the House of Representatives in protest.