The 20-year-old, netted in 2009 from Swiss outfit Grasshopper, has committed himself to the Marassi outfit until 2018.
Martinelli, a Switzerland youth international, spent the 2012-13 campaign on loan with Portogruaro in the Lega Pro.
“I’m happy with the contract extension,” he stated. “To continue being a part of Sampdoria is a dream which I have had for a while.”
His previous deal was set to expire in June 2014.