On 27 June 2013, Adams was selected with the 12th overall pick in the 2013 NBA draft by the Oklahoma City Thunder, becoming the first New Zealander to be selected in the first round of an NBA draft. He also became the first player out of Pittsburgh to be selected in the first round since 1999 when Vonteego Cummings was selected by the Indiana Pacers with the 26th overall pick. On 12 July 2013, he signed his rookie scale contract with the Thunder. In his NBA debut on 30 October 2013, Adams recorded two points, three rebounds, one assist and three fouls in 18½ minutes off the bench against the Utah Jazz. On 8 November, in just his fifth NBA game, Adams recorded 17 points and 10 rebounds in a 119–110 win over the Detroit Pistons. He failed to surpass either mark for the rest of the regular season. He appeared in 81 games (20 starts) and averaged 3.3 points, 4.1 rebounds (9th among NBA rookies) and 0.70 blocks (9th among NBA rookies) in 14.8 minutes. In Game 6 of the Thunder's second round playoff series against the Los Angeles Clippers, Adams recorded 10 points and a season-high 11 rebounds. The Thunder reached the Western Conference Finals, where they were defeated by the San Antonio Spurs in six games. At the conclusion of the 2013–14 season, Adams earned NBA All-Rookie Second Team honours.