Dlamini-Zuma, 68, is considered a leading candidate to replace Zuma as head of the ANC at a party conference in December.
Local media reported that Nkosazana Dlamini-Zuma, a former African Union chief, was sworn in Thursday as a member of parliament for the governing African National Congress.
She posted on Twitter that "5 years ago I left parliament with mixed feelings today I return as directed by (the ANC) as a servant of the SA (South African) people."
"I return determined to contribute to implementing decisions of (the ANC) particularly as it relates to radically improving our people's lives," she wrote.
Dlamini-Zuma, 68, is considered a leading candidate to replace Zuma as head of the ANC at a party conference in December, though she has not publically declared her candidacy in the leadership battle.
But she has been endorsed by the ANC's women and youth leagues, and enjoys support from the powerful KwaZulu-Natal province, which is also Zuma's stronghold.
Since she left the AU early this year, she has been campaigning across the country, and nominations for the party leadership posts are expected to be announced this month.
Zuma is widely seen as favouring Dlamini-Zuma, with whom he has four children, ahead of Deputy President Cyril Ramaphosa, which would set up a showdown that could split the ANC.
Dlamini-Zuma last month rejected criticism that her bid to succeed Zuma was a strategy to protect the embattled leader from criminal prosecution once he leaves office, telling AFP that she found such allegations "offensive".
President Zuma, who completes his final term in power in 2019, faces a slew of court cases, including one stemming from nearly 800 corruption charges that may be reinstated over a multibillion-dollar arms deal in the 1990s.