Under the deal, Mr Saleh will retain the title of Yemeni president but will not have any powers until elections are held in January, opposition officials say.
Saleh in Riyadh to sign GCC-brokered exit plan: reports
"The signing ceremony will take place today in Riyadh," Jamal Benomar said, confirming that the plan, signed by the opposition in April, will now be inked by the veteran leader himself after months of stalling.
Both parties will also sign a UN-crafted road map which sets a mechanism for implementing the Gulf plan, under which Mr Saleh will hand power to his deputy in return for immunity from prosecution.
Under the deal, opposition officials said Mr Saleh would retain the title of president, but would not have any powers, until elections were held in January.
Mr Saleh has backed down three times from signing the initiative, which calls for him to hand over his power to his vice president, Abdrabuh Mansur Hadi, in return for immunity from prosecution for himself and his inner circle.
Yemeni television said Saleh was already in Riyadh for the signing ceremony.
His trip to the Saudi capital "comes in response to an invitation by the Saudi leadership to attend the signing of the Gulf Initiative and its implementation mechanism ... to take the country out of its crisis," it said.
* With reporting by Agence France-Presse and Mohammed Al Qadhi