Testaverde's signing came on the same day the Panthers placed quarterback Jake Delhomme on injured reserve, ending his season. Delhomme is to have ligament-replacement surgery on his right arm next week.
Delhomme's replacement as the starting quarterback, David Carr, suffered a back injury last week and Carr's availability for this week's game at Arizona is undetermined.
Testaverde, 43, is a 20-year NFL veteran who ranks sixth in NFL history in pass attempts (6,529), completions (3,693) and passing yards (45,281). He has thrown 270 touchdown passes, ninth-most in league history.
He previously played for Tampa Bay, Baltimore, the New York Jets, Dallas and New England. He appeared in three games for the Patriots last year and was with the team in training camp this year.