Booker plans to rally with Biden in Flint and Detroit in Michigan later Monday with Senator Kamala Harris.
Harris endorsed Biden on Sunday.
Booker released a statement on his Facebook page with the announcement on Monday morning.
Booker ended his own presidential bid in January, pledging to do "everything in my power to elect the eventual Democratic nominee for president."
"The answer to hatred & division is to reignite our spirit of common purpose," Booker tweeted.
Vermont Senator Bernie Sanders has been endorsed by Marianne Williamson and New York City Mayor Bill de Blasio.
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