Pew Internet:

For the first time since the Pew Research Center’s Internet & American Life Project began systematically tracking smartphone adoption, a majority of Americans now own a smartphone of some kind. Our definition of a smartphone owner includes anyone who says “yes” to one—or both—of the following questions:

55% of cell phone owners say that their phone is a smartphone.
58% of cell phone owners say that their phone operates on a smartphone platform common to the U.S. market.1

Taken together, 61% of cell owners said yes to at least one of these questions and are classified as smartphone owners. Because 91% of the adult population now owns some kind of cell phone, that means that 56% of all American adults are now smartphone adopters. One third (35%) have some other kind of cell phone that is not a smartphone, and the remaining 9% of Americans do not own a cell phone at all.