The 20-year-old joined the Toffees aged 10, progressed through the academy and became a regular for the Under-23s.
Kiersey becomes Darrell Clarke's fifth signing for the Saddlers, after the arrivals of Stuart Sinclair, James Clarke, James Hardy and Elijah Adebayo.
"I want to play league football," Kiersey said.
"It's a big club and Darrell Clarke is a good manager with young players, so I was eager to get here."
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