Thomas, 75, is the oldest and longest-serving member of the court and is part of its 6-3 conservative majority. He was appointed to the top U.S. judicial body in 1991 by Republican President George H.W. Bush.
Chief Justice John Roberts on Monday noted that Thomas would not appear on the bench for the arguments but gave no reason for the absence. Roberts said Thomas would "participate fully" in the cases by using the written legal briefs and the transcripts of the arguments.
A court spokesperson did not provide additional information on Monday's absence.
The court often gives a reason for a justice's absence, including illness.
The court on Tuesday was hearing a high-profile dispute - a Pennsylvania man's bid to avoid an obstruction charge related to the Jan. 6, 2021, attack on the Capitol - a case with possible implications for the federal prosecution of Donald Trump for his efforts to overturn his 2020 election loss.
Thomas previously missed arguments in March 2022 when he was hospitalized with flu-like symptoms and diagnosed with an infection.
(Reporting by Andrew Chung in New York and John Kruzel in Washington; Editing by Will Dunham)
