The presumptive Democratic presidential nominee appeared for the first time in two months to lay a wreath near his Delaware home.
Mr Trump retweeted a post from Fox News political analyst Brit Hume, who shared a close-up shot of Mr Biden wearing the mask.
“This might explain why Trump doesn’t like to wear a mask in public. Biden today,” it read.
Mr Biden, 77, and his wife Jill covered their faces as they laid the wreath at the veterans park.
Mr Trump, 73, chose not to wear a mask as he attended events at Arlington National Cemetery and Fort McHenry in Baltimore, despite advice from the Centers for Disease and Prevention for Americans to wear them.