Mandhana, the 22-year-old opener, topped the KSL batting charts last year with 421 runs at an average of 60.14 and a strike rate of 174.68, hitting a century and two half-centuries, helping Storm reach Finals Day before having to return for India duties.
"I enjoyed my time with Western Storm last year and I'm looking forward to coming back in 2019. We played some outstanding cricket last year and hopefully we can go a little further this season and lift the trophy," she said in a statement on the team's website. "Everyone involved with Western Storm made me feel so welcome and, if given the opportunity, I was always going to come back.
"The culture around the team, from top to bottom, was very good and I know that everyone will be looking for success once again. The squad has a good mix of experienced players plus young players who have a point to prove. I hope that I can score the runs that will help to contribute to a successful year for Western Storm."
Speaking about the signing, Storm head coach Trevor Griffin said, "Everyone knows what Smriti is capable of and she was absolutely sensational for us last year. Her performances on the field were superb and really set the tone for what we were trying to achieve in each match.
"It wasn't just the volume of her runs that was impressive, it was the manner in which they were scored that really caught the eye. She was exceptional off the field as well. Her team ethic and the way in which she goes about her training will no doubt inspire some of our younger players, who will certainly learn from someone like her."
Storm recently announced re-signing the England quartet comprising Heather Knight, Anya Shrubsole, Fran Wilson and Freya Davies, and the New Zealand wicketkeeper-batsman Rachel Priest, who have all been part of the franchise since the inaugural season, held in 2016.