The move, backed by her husband former president Jerry Rawlings, threatens to split support for incumbent John Mahama, who is seeking re-election against his main opposition rival Nana Akufo-Addo on December 2.
“I stand before you today because I chose to become bolder and braver, and together we are going forward,” Agyeman-Rawlings said in a speech broadcast on the radio.
She had tried to win the ruling NDC party’s nomination last year, but lost to then-President John Atta Mills, who died in July and was replaced by Mahama.
Ghana, a cocoa, oil and gold producer, is one of West Africa’s most stable countries, with more than 20 years of political transitions through elections following a flurry of coups.
Jerry Rawlings, who still enjoys broad popularity, said his wife left the ruling NDC because of bad governance.
“The NDC leadership had lost its moral high ground, (and was) involved in things that it should not have gotten into, corruption at the leadership level, etc,” he said.
Agyemang-Rawlings, 63, is a women’s rights advocate and is believed by many to have played an influential role during her husband’s rule.