"I pray for him, for God to help him," she said during an improvised press conference in her garden.
"It is a tough race, so I have prayed for him. If this is his turn (to win again), God will let him triumph," she added in her native Dholuo language.
Sarah Obama, who turns 90 this year, is the third wife of Barack Obama's paternal grandfather. She is not related by blood to the US president, but he considers her -- "Mama Sarah" -- his grandmother.
In the run-up to the election, Sarah Obama has mostly remained indoors under 24-hour police protection.
Kogelo, nestled in hills some 60 kilometres (40 miles) from Kisumu -- the main town in western Kenya -- has undergone a major facelift since Obama's election four years ago.
The main road has been tarmacked, and residents have electricity and running water.