In Washington, Porziņģis bounced back, but was traded to the Boston Celtics in 2023 as part of the Wizards' front office's desire to rebuild.

[4] In 2010, Baloncesto Sevilla, a club that had a professional team competing in the Liga ACB in Spain, called Porziņģis for a tryout in an attempt to recruit foreign talent to its junior squads.

"[4] Inmaculada Avivar, a nutritionist for Sevilla, diagnosed that Porziņģis was suffering from anemia, meaning that he had fewer red blood cells.

[4] In addition to medical problems, Porziņģis had trouble picking up the language in Seville, making him reluctant to return there after his first tryout.

In his first season playing for the junior squad, Porziņģis struggled to communicate with the coaching staff and his teammates and was often tired due to his health.

[11] Two days later in the team's tournament finale vs Real Madrid's youth squad, Porziņģis scored 24 points and grabbed 11 rebounds.

Entering the 2012–13 ACB season, Sevilla had hired Aíto García Reneses, who previously worked with the likes of Pau Gasol, Juan Carlos Navarro, Ricky Rubio and Rudy Fernández.

[15] Porziņģis opened his second season on Sevilla's main squad strong in his third game against the EuroLeague team Laboral Kutxa, leading them to a 20-point ACB victory on 2 November 2013.

[18] Before and after he made the decision, Porziņģis drew interest from National Basketball Association (NBA) teams such as the Orlando Magic, who held the 12th overall pick, and traveled to Europe to scout him.

Porziņģis's agent Andy Miller released the information to ESPN, saying that his client did not feel prepared to become a part of the NBA and wanted to develop his skills until the 2015 draft.

However, Porziņģis had a strong EuroCup performance against Pallacanestro Virtus Roma in the weeks that followed, in which he contributed 18 points, seven rebounds, four assists, four steals and two blocks.

[26] Porziņģis was compared with players such as Pau Gasol and Dirk Nowitzki, but also Darko Miličić, a former lottery selection widely considered a draft bust.

Sports wrote, "Porziņģis has an innate awareness about the way the American public sees a young, long European teenager.

[31] On the same night, the Knicks traded for Porziņģis' teammate from Sevilla, Willy Hernangómez, who was originally drafted by the Philadelphia 76ers as a 35th overall pick.

[34] On 21 November, Porziņģis had 24 points, 14 rebounds, and seven blocked shots to lead the Knicks to a 107–102 victory over the Houston Rockets, becoming the first 20-year-old to post such a stat line in a single game since Shaquille O'Neal in 1992–93,[35] and the first rookie to reach those totals since Tim Duncan in 1998.

[46] On 19 January 2017, Porziņģis came off the bench for the first time in his NBA career after returning from a four-game absence due to a sore left Achilles tendon.

[51] He was subsequently named Eastern Conference Player of the Week for games played from Monday, 30 October through Sunday, 5 November.

[55] However, on 6 February 2018, in a 103–89 loss to the Milwaukee Bucks,[56] Porziņģis tore his left ACL and was subsequently ruled out for the rest of the season.

On 31 January 2019, after a meeting with the Knicks left franchise officials with the impression that Porziņģis wanted to be traded,[59] he was traded alongside Trey Burke, Courtney Lee and Tim Hardaway Jr. to the Dallas Mavericks in exchange for DeAndre Jordan, Wesley Matthews, Dennis Smith Jr., an unprotected 2021 first-round draft pick, and an additional top-ten protected 2023 first round draft pick.

[69] Three days later, he had 34 points, 12 rebounds, and five blocks in a 127–123 overtime victory over the New Orleans Pelicans, becoming the first player since Shaquille O'Neal in 2000 to have consecutive 30-point, five-block games.

Porziņģis became just the sixth player in franchise history to have multiple seasons averaging over 20 points per game, joining Mark Aguirre, Rolando Blackman, Luka Dončić, Michael Finley and Dirk Nowitzki.

On 10 February 2022, the Mavericks traded Porziņģis and a protected 2022 second-round pick to the Washington Wizards in exchange for Spencer Dinwiddie and Dāvis Bertāns.

On 23 June 2023, the Wizards traded Porziņģis to the Boston Celtics as part of a three-team deal that sent Marcus Smart to the Memphis Grizzlies and Tyus Jones to Washington.

[119] During his rookie season, Kevin Durant dubbed Porziņģis a basketball "unicorn" because of his rare combination of size and skills.

Due to his height and mobility, Porziņģis is able to shoot over most defenders, with his sheer size and high volume creating a unique advantage initially shared by no other player in the NBA.

[122] More recently, younger "unicorns" like Victor Wembanyama and Chet Holmgren have entered the league with playing styles often compared to Porziņģis.

[124] In a 2017 E:60 documentary on Porziņģis' life, his parents revealed that they had another son, Toms, who was born four years before Kristaps and died at the age of 14 months.

[130][131] Porziņģis is a football fan and supports his hometown team FK Liepāja, as well as Real Madrid and former club Sevilla.

[134] In March 2019, it was publicly revealed that a woman accused Porziņģis of raping her at the Sky building in February 2018, hours after he tore his ACL.

Porziņģis' attorney, Roland G. Riopelle, publicly denied the claim, and said he had previously referred the case to federal authorities due to the "accuser's extortionate demands.