The move, by unanimous consent of the 100 senators, serves as a check on Trump as he mulls firing his Attorney General Jeff Sessions.
Trump has considered replacing his embattled top law enforcement official amid broadening investigations into allegations that his aides coordinated with Russia to tilt last year's election in his favor.
The president has strongly criticized Sessions for recusing himself from the probe, and speculation swirled over whether Trump would ram through a replacement during the break, without Senate confirmation.
Such sessions typically last a minute or two, with no business conducted. Holding them keeps the Senate open during the vacation period, denying the president recess appointment opportunities.
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The House of Representatives adjourned for its break last week.
As summer-bound US lawmakers exit Washington, the numbers are telling: with Trump in office seven months, and his Republican Party controlling all three branches of government, Congress has passed zero pieces of major legislation.
The Republicans' over-promised effort to repeal and replace Barack Obama's health care reforms collapsed in embarrassing shambles.
But the health care fiasco sucked up weeks of debate time, and its cascading effect stalled other legislation.
Republicans do claim some victories, including confirming conservative judge Neil Gorsuch onto the Supreme Court, rescinding several Obama-era regulations and improving conditions for veterans.
One top accomplishment was the Russia sanctions bill that passed with overwhelming support, leading Trump to sign the measure into law despite not backing it.
But otherwise they now head home with few accomplishments, and likely to face conservatives angry at their failure to repeal Obamacare.
Number two Senate Republican John Cornyn signaled the health fight was not over.