In it he offered Donald Trump, the incoming President, his congratulations, a rather pointed guide to the American constitution and some words of advice.
“Congratulations on a remarkable run,” he wrote, according to a copy of the letter obtained by CNN. “Millions have placed their hopes in you, and all of us, regardless of party, should hope for expanded prosperity and security during your tenure.
“This is a unique office, without a clear blueprint for success, so I don’t know that any advice from me will be particularly helpful. Still, let me offer a few reflections from the past 8 years.”
Perhaps as a gentle rebuke to Mr Trump’s fiery rhetoric of building walls and putting America first, Mr Obama urged his successor to build “ladders of success” for people less fortunate and to remember the importance of American leadership in the world.
But perhaps most pointedly he reminded the 45th president – a political neophyte – of his place in the constitution and the temporary nature of his time in the White House.
“That makes us guardians of those democratic institutions and traditions – like rule of law, separation of powers, equal protection and civil liberties – that our forebears fought and bled for. Regardless of the push and pull of daily politics, it’s up to us to leave those instruments of our democracy at least as strong as we found them,” he wrote.
In his initial days in office, Mr Trump thanked his predecessor for wise counsel during the run-up to the inauguration.
When George W Bush left, he wrote to Mr Obama: “There will be trying moments. The critics will rage. Your ‘friends’ will disappoint you. But, you will have an Almighty God to comfort you, a family who loves you, and a country that is pulling for you, including me.”
Mr Obama struck a similar tone as he encouraged Mr Trump to find comfort in the people close to him.
“And finally, take time, in the rush of events and responsibilities, for friends and family. They’ll get you through the inevitable rough patches,” he wrote.