His endorsement came after meeting Vermont Senator Bernie Sanders who has been battling Mrs Clinton for the nomination.
Speaking in a video tweeted by Mrs Clinton, Mr Obama said she may be the most qualified person “ever” for the role of president.
Leading liberal politician Elizabeth Warren also endorsed Mrs Clinton.
“I am ready to get in this fight and work my heart out for Hillary Clinton to become the next president of the United States,” the Massachusetts senator told MSNBC, “and to make sure that Donald Trump never gets any place close to the White House.”
President Obama and Mrs Clinton are set to start campaigning together soon.
“I want those of you who’ve been with me from the beginning of this incredible journey to be the first to know that ‘I’m with Her.’ I am fired up and cannot wait to get out there and campaign for Hillary,” Mr Obama said in the video.
“Secretary Clinton and Senator Sanders may have been rivals during this primary, but they’re both patriots who love this country and they share a vision for the America that we all believe in.”