The 35-year-old has been training with Quakers for a month after leaving previous club Glentoran.
Darlington manager Mark Cooper: “Keith’s a clever player, he can keep hold of the ball and create things and he’s still got a little bit of pace, so it was a no-brainer for us to sign him.
“He’s signed a one-month deal to start with and we’ll take it from there.
“He’s been training and constantly having to prove his worth and he’s done that.
“Keith knows the game inside out and he will be a good influence around the place and, hopefully, help us keep the run going.”
Darlington become Gillespie’s eighth club since leaving St James’ Park in 1998.