Rice was drafted in the fourth round of the November 2, 2012 NBA Development League draft by the Rio Grande Valley Vipers. He made the opening day roster and made his professional debut on November 23 against the Bakersfield Jam. Rice did not play much early in the year, totalling only 147 minutes in the first 22 games. On February 4, Rice posted professional career highs of 35 points and 15 rebounds against Springfield Armor. The day before the 2013 NBA All-Star Game, he participated in the 2013 D-League Dream Factory Dunk Contest. On March 5, Rice posted 20 points, 14 rebounds and 4 assists against the Iowa Energy. On March 6, he posted 29 points, 11 rebounds and 3 blocks against the Austin Toros. On March 10, he posted 23 points, 8 rebounds, 3 assists and a steal against the Reno Bighorns. On March 11, Rice was named D-League Player of the Week after averaging 24.0 points, 11.0 rebounds, and 2.3 assists in three victories that week. Following the regular season, Rice was named 2012–13 NBA D-League All-Rookie second team. Rice emerged as a starter and helped the Vipers win their final 16 consecutive games, including three consecutive D-League playoff series sweeps on the way the 2013 NBA D-League championship. Rice averaged 25 points, 9.5 rebounds, 4.3 assists, 2 blocks and 2 steals in the playoffs including averaging 29 points, 11.5 rebounds, 4 assists, 3 steals and 3.5 blocks in the D-League finals.