The 20-year-old Stephens, who upset Serena Williams in the Australian Open quarterfinals to start 2013, has improved 26 places in the rankings to No. 12 this year.
Isner won the 2011 Hopman Cup with Bethanie Mattek-Sands and combined with Venus Williams for the 2013 edition.
Under the competition format, eight teams are split into two groups for matches including men's and women's singles and mixed doubles. The winners of each round-robin group advance to a final.
Anabel Medina Garrigues helped Spain win the title last year and is returning for the defense, when she'll be partnered by two-time Hopman Cup champion Tommy Robredo.
Former U.S. Open champion Sam Stosur and Bernard Tomic will represent Australia, while Jo-Wilfried Tsonga and Alize Cornet will play for France.
Poland will make its debut in the tournament with a team containing No. 4-ranked Agnieszka Radwanska and Wimbledon semifinalist Jerzy Janowicz.
U.S. Open semifinalist Andreas Seppi will join Flavia Pennetta in the Italian team, and Milos Raonic will team up with Eugenie Bouchard for Canada.