LONDON—Helios Investment Partners LLP, a private-equity firm that focuses on Africa, has bought a minority stake in Nigeria's ARM Pension Managers PFA Ltd. to tap into the nation's fast-growing pension market, according to co-founder Tope Lawani.

Helios invested about $50 million, according to a person familiar with the situation. Nigeria's pension market has grown by 30% a year since 2006, when a pension law requiring mandatory contributions was implemented, according to Helios. ARM manages about $2.1 billion.

"We believe, conservatively, that aggregate defined contribution pension assets in Nigeria will exceed $150 billion within the next 10 years, representing a sevenfold increase," Tope Lawani, the former TPG Capital executive who co-founded Helios in 2004, said in an interview.

Nigeria's mandatory defined-contribution pension plan should double registrations to 13 million by 2020, Bank of America Merrill Lynch analysts, including Wesley Fogel, wrote in a note last year. "This is a new industry which has grown from nothing in 2004 when the pension legislation was approved," Mr. Fogel told The Wall Street Journal.

Helios's previous investments include Nigerian payment processing company Interswitch and Kenya's Flamingo Holdings, which grows cut flowers and fresh vegetables.

Write to Simon Clark at simon.clark@wsj.com