Ayton made his NBA debut in the season opener on October 17, 2018 and earned a double-double with 18 points and 10 rebounds, as well as a season-high 6 assists, a steal, and a block with 36 minutes played as a starter in a 121–100 win over the Dallas Mavericks. It made Ayton the first player to record these stats in their debut game since Lew Alcindor in 1969, as well as the third player since Oscar Robertson in 1960. Five days later, Ayton recorded a double-double of 20 points and 14 rebounds (alongside 5 assists) in a 123–103 loss to the Golden State Warriors. On October 27, he recorded 24 points on near-perfect shooting (12/13 shots made) alongside 5 assists in a loss to the Memphis Grizzlies. He became the second rookie in 40 seasons (first being Adam Keefe) to score at least 12 shots with a 90% or higher field goal percentage. Six days later, Ayton recorded a double-double of 17 points and a season-high 18 rebounds in a 107–98 loss to the Toronto Raptors. Ayton later recorded a new career high of 33 points, alongside 14 rebounds, in a 122–118 loss to the Denver Nuggets on December 29. Through the first half of the season, Ayton started in every game for the Suns when he was healthy, save for one. On January 29, 2019, Ayton was named a member of the World Team for the Rising Stars Challenge in the 2019 NBA All-Star Weekend. On March 27, Ayton broke Alvan Adams' rookie record for most double-doubles recorded by a rookie in franchise history. However, Ayton sprained his left ankle three days later against the Memphis Grizzlies, ending his rookie season early. He became the third rookie in a decade to average a double-double in their rookie season, joining Blake Griffin and Karl-Anthony Towns. At the conclusion of the season, Ayton was named as one of three finalists for the NBA Rookie of the Year Award in the 2019 NBA Awards show. On May 21, Ayton was selected to the 2019 All-Rookie First Team.