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.