Provan, who can also play as a winger, left Notts' fellow Women's Super League One club Reading at the end of the 2016 WSL season.
She said: "I'm very excited to join such an experienced squad."
Manager Rick Passmoor added: "Shelly adds a wealth of knowledge. She gives us a different option on the left-hand side, in an attacking sense as well."
The Spring Series sees teams play each other once in a one-off, six-week transitional competition before the WSL's switch to a winter calendar.
For top-flight teams, the Spring Series runs over six weeks, from 23 April until Saturday, 3 June.
Notts County Ladies have signed full-back Shelly Provan on a contract until the end of the 2017 Spring Series.