The 37-year-old soccer star and his fashion designer wife will christen the 10-month-old later this year and have asked Longoria to play an important role in their little girl's life, reported Mirror online.
"Victoria asked me to be Harper's godmother and of course I said yes. I'm absolutely thrilled. Harper is simply adorable," she said.
Victoria and Eva became close friends since the Beckhams moved to Los Angeles in 2007 and she recently described Harper as a "chunky monkey".
David and Victoria, who also have sons Brooklyn, 13, Romeo, nine, and seven-year-old Cruz together, will baptise Harper in a lavish ceremony at their UK home and guests are expected to include Tom Cruise, Katie Holmes and Will Smith.
"David and Victoria feel it's important Harper is christened. And they wanted the ceremony in the UK," a source said.