JOHANNESBURG—Rwanda and Nigeria are Africa’s most promising markets for global retailers, according to consulting firm A.T. Kearney, as foreign investors look at all corners of a sprawling continent where a new middle class is expected to keep growing.

“There are opportunities all over Africa,” said Mike Moriarty, a partner at A.T. Kearney and lead author of the firm’s African Retail Development Index, a ranking of some 20 countries the firm has identified as attractive to global retailers.

At the top of the list, Rwanda and Nigeria present very different opportunities and risks.

Rwanda is a small, landlocked nation of 11 million people, most of whom are poor. But under the leadership of longtime President Paul Kagame, Rwanda has developed a reputation as being one of the African countries most welcoming to foreign companies.

Nigeria, on the other hand, is a nation of some 170 million people. Many of them are destitute, but millions are making enough of an income to acquire a taste for branded foreign goods. That growing potential can make it worth braving Nigeria’s unpredictable politics and security risks, Mr. Moriarty said.

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