Williams battled a pulled abdominal muscle last year and had not won a title in 14 months. She missed the last six months of the 2003 season with the ailment and struggled in her comeback, failing to get past the quarterfinals in four previous tournaments.
But the former world No. 1 became only the 13th player in the Open Era to win 30 career titles and tied Tracy Austin for 11th place on the career list.
Williams earned $189,000 for her first title since the Proximus Diamond Games at Antwerp in February 2003. It is her first on clay since Amelia Island in 2002.
The 32-year-old Martinez won this event in 1994 and 1995.