The agreement, already signed by five other Nile River countries, was unanimously endorsed by Ethiopia's 547-member parliament today.
The vote comes amid tensions between Ethiopia and Egypt over Ethiopia's Nile dam project that is diverting the Nile waters to a massive hydro-electric dam.
Egypt fears the dam will result in a diminished share of the Nile, which provides most of the desert nation's water needs.
Egyptian President Mohammed Morsi on Monday warned that "all options are open" to challenge Ethiopia's project.
