Speaking on NBC's "Meet the Press," the Nevada Democrat minced no words when asked about Thomas being a possible replacement for Chief Justice William Rehnquist.
"I think that he (Thomas) has been an embarrassment to the Supreme Court," Reid said. "I think that his opinions are poorly written. I just don't think that he's done a good job as a Supreme Court justice."
Rehnquist announced he has thyroid cancer, which triggered speculation about his replacement.
Reid was less critical of Thomas' fellow conservative, Justice Antonin Scalia, CNN reported.
"I cannot dispute the fact, as I have said, that this is one smart guy," Reid said. "And I disagree with many of the results that he arrives at, but his reasons for arriving at those results are very hard to dispute."
Reid also warned Republicans against efforts to override the Democrats' right to filibuster against judicial appointments, the Washington Post said.
"During the past four years ... we have approved 207 federal judges and turned down 10," he said. The president, he said, "should be happy with what he's gotten."