The Prime Minister insisted that the UK-US so-called special relationship would "continue to prosper" with the President-elect.
Sir Keir is among leading Labour politicians who have been critical of the Republican in the past.
He also faces a challenge to mend fences after a row over Labour staffers heading across the pond to campaign for Democrat Kamala Harris.
The PM, who once declared himself to be "anti-Trump but proAmerican", rushed to congratulate the 78-year-old on his "historic election victory" after US voters returned him to the White House.
Sir Keir said: "I look forward to working with you in the years ahead.
"As the closest of allies, we stand shoulder to shoulder in defence of our shared values of freedom, democracy and enterprise. From growth and security to innovation and tech, I know the UK-US special relationship will continue to prosper on both sides of the Atlantic."
Last night he spoke to the President-elect and "said he looked forward to working closely with President-elect Trump across all areas of the special relationship...from defence and security to growth and prosperity", Downing Street said.
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