Heller is the 25th senator from Nevada and will hold the seat for 18 months before the term expires. Heller, who was serving his third term in the House, announced earlier this year that he will run for Ensign's seat in 2012.
Nevada Gov. Brian Sandoval (R-Nev.) appointed him to fill out the remainder of Ensign's term.
After Heller was sworn in, Senate Majority Leader Harry Reid (D-Nev.) welcomed him to the Nevada delegation in the Senate.
"I look forward to working with our new junior senator to make the tough choices that will help our state and nation recover [from the recession]," said Reid.
Reid is holding a reception for Heller in the Capitol later Monday afternoon.