The rare visit to Kenya, Uganda and Zimbabwe marks the latest diplomatic effort by the Islamic Republic to forge new alliances and ease its international isolation. The three-day trip will be the first by an Iranian president to Africa in 11 years.
Raisi was originally scheduled to hold a joint press conference with Kenyan President William Ruto on Tuesday morning, but neither showed up.
"The schedule of the president has now been reviewed to allow for finalization of key MoUs (memoranda of understanding) that are central to the furtherance of relations," Kenya's foreign ministry said in a statement, adding: "The Iranian president will now arrive tomorrow for a state visit."
According to Iran's official IRNA news agency, Raisi will head a delegation that includes the foreign minister as well as senior businesspeople. He is scheduled to meet with presidents from the three countries.
On Monday, Iranian foreign ministry spokesman Nasser Kanani described the trip as "a new turning point" which could bolster economic and trade ties with African nations. He also said the rapprochement is based "on common political views" between Tehran and the three African countries.
On Saturday, Raisi welcomed Algerian Foreign Minister Ahmed Attaf in a bid to boost relations with Algiers.
Last week, Iran became a member of the Shanghai Cooperation Organization (SCO), which includes Russia, China and India.
In March, Iran agreed to restore ties with its regional rival Saudi Arabia under a China-mediated deal. It has since been looking to re-establish ties with other countries in the region including Egypt and Morocco.
In June, Raisi set out on a Latin America tour that included Venezuela, Nicaragua and Cuba before a trip to Indonesia.