On Twitter, Clinton wrote she's at the inauguration to "honor our democracy."
Clinton may be making a statement with her outfit choice today, wearing white, the color of the suffragette movement.
The 2016 Democratic presidential nominee later attended President Donald Trump's luncheon at the Capitol, where she shook hands with the 45th president.
This was the first contact the two rivals have had since trading jabs at the Alfred E. Smith Dinner held in New York City in October.
President Donald Trump made brief remarks at the luncheon and thanked the Clintons.
"I was very honored, very, very honored when I heard that President Bill Clinton and Secretary Hillary Clinton was coming today," he said to the room, which filled with applause.
"There's nothing more I can say because I have a lot of respect for those two people, so thank you all for being here," Trump said.