His retirement, which takes effect on July 31, gives Trump a second Supreme Court appointment in his 17 months in office after the Republican president past year selected Neil Gorsuch, who has already become one of the most conservative justices.
In the spring of 2016, before his election, Trump issued a list of 11 conservative judges he considered "representative of the kind of constitutional principles I value" and saying he would use it as "a guide to nominate our next United State Supreme Court justices".
The retirement of Mr Kennedy, a conservative who sided with liberals on many votes, gives Mr Trump the chance to shift the balance of the top court further to the right for decades to come.
There are only nine justices, who only get replaced if they retire or die, so who sits on it really matters.
Trump and Putin to meet in third country mid-July
Subsequent sanctions have punished Moscow for what Washington has called its malign behavior and meddling in US politics. President Trump is planning to hold a summit meeting in the near future with Russian President Vladimir Putin .
The very thought of this will dismay Democrats.
Trump's first high court nominee, Justice Neil Gorsuch, was confirmed in April 2017. "Anthony Kennedy kept the court up to date with where society was moving". "No question about it", McConnell said.
"I could see Trump appointing [Rudy] Giuliani, just to keep him off television", Colbert said.
Democratic Sen. Richard Blumenthal told reporters that members of his party will have a voice and a vote.
"We will be looking at the qualifications, the personal background of any nominee", Blumenthal said. "And the consequences for our nation couldn't be higher".
"The country is moving in the direction of supporting it - that's where history is taking us", she said. "No nominee should be rammed through without due consideration and debate".