Obasanjo will arrive in Zimbabwe 10 days before the vote, which has already been overshadowed by allegations of political intimidation and electoral rolls packed with "ghost voters".
In a statement on Friday, the African Union (UN) said the monitors – drawn from African non-profit organisations and member countries – will work with nine observers already on the ground.
Obasanjo has twice ruled Nigeria.
A former soldier, he headed a military government between 1976 and 1979 and served an eight-year term as an elected president between May 1999 and May 2007.
President Robert Mugabe has opposed the entry of non-African observers to monitor the election, which will see the long-term leader try to extend his 33-year term.
Rights groups have expressed concern that the vote will see a repeat of vote rigging and violence that has marred previous polls.
'Human rights violation'
Amnesty International on Friday called on regional bodies to "meticulously document human rights violations, in particular those committed by government agencies".
Zimbabwe's security forces, controlled by Mugabe (89), have in the past been accused of rights abuses and intimidating political opponents.
At least 200 people were killed in the run-up to the June 2008 presidential run-off between Mugabe and his arch rival Prime Minister Morgan Tsvangirai.
Tsvangirai will again be Mugabe's opponent on July 31. – AFP