The 22-year-old England Under-23 international will be eligible for Wednesday's match against Bristol City.
Walsh was available after Notts folded on 21 April, and Yeovil needed a keeper after first-choice Charlotte Haynes was ruled out for eight weeks.
"What better way to get stuck in than the derby," she told the club website.
Women's Super League One club Yeovil Town Ladies have signed former Notts County goalkeeper Megan Walsh for the rest of the 2017 WSL 1 Spring Series.