The 25-year-old centre back has 15 caps for Nigeria. She played for England at youth level before switching allegiance and making her Nigeria debut in 2022.
"Grateful to have signed for Al-Ittihad," Plumptre said on messaging platform X, formerly known as Twitter. "Excited to start this journey alongside some incredible human beings.
"My journey of stepping into more of myself continues... It's more than football."
The Saudi women's top flight is gearing up for its second season, with eight teams vying for the title.
Saudi men's Pro League teams have been spending huge sums to attract some of the best players from European clubs, including Neymar, Cristiano Ronaldo and Karim Benzema.
Critics accuse Saudi Arabia of engaging in "sportwashing" in the face of heavy criticism of its human rights record and equality issues.
Saudi Arabia Crown Prince Mohammed bin Salman has introduced reforms allowing women greater control over their lives in recent years but men still retain a tight grip on power in the kingdom.
By Pearl Josephine, Reuters