During the event, Mr. Biden frequently cast his gaze down as if consulting notes.
But at one point, Mr. Biden closed his eyes, put his head down, and remained motionless for some 15 seconds or more, video of the event shows.
Eventually, the 77-year-old candidate appeared to be startled awake, and he jerked his head up, eyes blinking.
Mr. Biden’s campaign billed the event as a “Women’s Town Hall on Covid-19.”
Mr. Biden’s campaign did not immediately respond to questions about whether he nodded off during the Town Hall.
The video had been viewed well over a half-million times early Tuesday evening after several Twitter accounts posted it, usually with some humorous commentary.
— Caleb Hull (@CalebJHull) April 28, 2020
As much of the nation remains shut down in response to the novel coronavirus, which originated in Wuhan, China, Mr. Biden has frequently appeared on video from an office in his Delaware home.
During many of his appearances, he has fumbled for words even at times where he seems to reading a script.
Those stumbles and other issues have led some to speculate the Democratic Party may seek to replace Mr. Biden, but Mrs. Clinton gave no hint of unease with his candidacy — or noticing the apparent catnap — during her enthusiastic endorsement.