He says Obama, a Democrat, is electable and would bring people together.
Caldwell says his support is personal and not connected to his role as pastor of the 14,000-member church.
However, Caldwell told his congregation that Obama may pay a visit to his church.
Caldwell, whose relationship with the president began when Bush was governor of Texas, introduced Bush at the 2000 Republican National Convention and gave the benediction at both of his inaugurations.
Caldwell said told Bush of his decision to support Obama and that the president was "OK" with it, adding that the minister's presidential choice would not affect their relationship.
