Morgan Tsvangirai boycotted the government three weeks ago after accusing the country's president, Robert Mugabe, of not implementing the power-sharing agreement and sanctioning the harassment of opposition MDC supporters.
Zimbabwe's prime minister has ended his boycott of the country's unity cabinet, saying he wants to give the body time to work out its difficulties.
Tsvangirai's decision on Thursday followed a meeting in Maputo, Mozambique, with members of the Southern African Development Community (SADC).
"We have suspended our disengagement from the GPA (Global Political Agreement) with immediate effect and we will give President Robert Mugabe 30 days to implement the agreement on the pertinent issues we are concerned about," he told reporters.
He said his return to the cabinet would give the southern African group time to mediate.
Tsvangirai and Mugabe formed their coalition in February at the urging of the SADC.
Tsvangirai's Movement for Democratic Change (MDC) began boycotting cabinet meetings with Mugabe's ZANU-PF last month, although he said at the time that his withdrawal was only a temporary move.
The MDC leader has accused Mugabe of being a "dishonest and unreliable partner" for refusing to implement power-sharing fully, particularly regarding decisions over senior appointments.
Nonetheless Tsvangirai has said he sees the coalition as the only way to rescue Zimbabwe from economic ruin and political violence.
Mugabe's ZANU-PF party has accused Tsvangirai and the MDC of not doing enough to persuade Western nations to lift travel and financial sanctions targeted at ZANU-PF leaders and their business allies.
Western donors have said they are reluctant to send funds to Zimbabwe until a stable government creates a democracy that can implement political and economic reforms.
Morgan Tsvangirai
Morgan Tsvangirai, the prime minister of Zimbabwe, said on Thursday that he was calling off his boycott of the coalition government with president Robert Mugabe. The prime minister said he was giving Mugabe and his Zanu-PF party 30 days to work on "the pertinent issues we are concerned about".

Tsvangirai and his Movement for Democratic Change (MDC) party had pulled out of the coalition government three weeks ago, protesting what they called harassment of Mugabe opponents and accusing the president of not fulfilling the power-sharing agreement agreed to in February.

"We have suspended our disengagement from the GPA Global Political Agreement with immediate effect and we will give President Robert Mugabe 30 days to implement the agreement on the pertinent issues we are concerned about," Tsvangirai said. His move came after he attended a meeting in Mozambique with the Southern African Development Community.