In a statement on Tuesday, the long-time Republican lawmaker’s office said that he is being treated with antibiotic infusions but remains in “good spirits”.
“Senator Chuck Grassley (R-Iowa) is receiving antibiotic infusions at an area hospital to treat an infection,” the statement said.
“He is in good spirits and will return to work as soon as possible following doctors’ orders.”
No further details about his illness or condition are known at this time.
In 2023, the Republican lawmaker – who was long known for keeping fit with daily early morning runs – was briefly hospitalised last year to undergo hip surgery.
He is also the longest-serving Republican in US Senate history, having held his seat for more than four decades since first being elected in 1980. He is also the sixth-longest-serving US senator in American history.
His absence from the Senate comes as Congress is hurtling towards Friday’s deadline to pass a stopgap spending bill to avert a government shutdown.
Senate majority leader Chuck Schumer and House speaker Mike Johnson have reached a tentative deal – but it still needs to pass formal votes in both chambers.
On Tuesday night, Congress took a key first step to passing the continuing resolution before Friday, with the Senate voting 68-13 to advance it.
Mr Schumer said on Tuesday that he expects the continuing resolution to be wrapped up “no later than Thursday”.
The stopgap bill will extend government funding through to 1 March and 8 March, to allow for more time to pass longer-term funding.
If the continuing resolution fails to pass by Friday, government funding will run out for some federal government agencies – and the government will partially shut down.