The 34-year-old Italian had been training with the club in recent weeks before agreeing to the deal. He was most recently with Sampdoria, where he spent two seasons in Serie B before leaving in the summer.
“It’s been great,” Borini said. “I came in to train to keep myself fit in this transition between contracts and Salford, and the gaffer, were kind enough to allow me to use the facilities and train here.
“Experience is one thing I can bring, but it’s not just what I have on the pitch – it’s also off it, leading by example in training, outside training, and learning how to manage a game.
“I’ve never played League Two football, so I’ll need to learn from my teammates about the main characteristics of this league. But I’ll bring myself as an open book – to help, to learn, to contribute. It’s a challenge I’m embracing because I needed real football again – something that warms my heart.”
Borini began his career at Bologna before joining Chelsea’s academy in 2007. After a successful spell with Roma, he moved to Liverpool in 2012, later finding regular football at Sunderland.
He went on to play for AC Milan, Hellas Verona and Turkish side Fatih Karagümrük before joining Sampdoria.
Borini is expected to be available for Salford’s next fixture, away to Oldham Athletic on 18 October.