An officer saw the star get out of his black Bentley around 4:30 a.m. and urinate in the street, police spokesman Bobby Hernandez said. The officer then approached him, noticed signs of intoxication and arrested him, Hernandez said.
Willis, 24, refused a breath test at a police station and was being transported to Miami-Dade County jail, Hernandez said.
Marlins officials and Willis' agent, Matt Sosnick, did not return phone messages seeking comment.
Willis finished last season 12-12 with a 3.87 ERA in 223 1/3 innings. The year before, he led the major leagues with 22 wins. He was the National League rookie of the year in 2003, the same year the Marlins beat the New York Yankees in the World Series.