Porziņģis followed his parents' footsteps and started to play basketball at age 6. Once he turned 12, his older brother, Jānis, who played professionally in Europe, would take Porziņģis to offseason training sessions. He played in youth competition with BK Liepājas Lauvas, the most famous club based in hometown of Liepāja, until he was 15 years old. An agent from Latvia sent video of him around this time to teams in Spain and Italy. 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. He stood 6 ft 8 in (2.03 m) and weighed 157 lbs (71 kg) at this time. Porziņģis recalled the moment and said, "I came here with my brother for two or three days, but it was really hot and I couldn't play at my best because of that. Still, I received a contract in summer 2010 and I signed it." Inmaculada Avivar, a nutritionist for Sevilla, diagnosed that Porziņģis was suffering from anemia, meaning that he had fewer red blood cells. This caused him to feel fatigue, shortness of breath, and inability to exercise. Nevertheless, he gradually overcame the condition and began seeing improvement in his game. In addition to his 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, he struggled to communicate with the coaching staff and his teammates and was often sleepy due to his health.
Porziņģis debuted for the youth squad on January 4, 2012, against its counterpart from Barcelona, adding 12 points and 10 rebounds. The opposing side was led by Alexandr Zhigulin, who would go on to enter the 2015 NBA draft and withdraw. However, Sevilla was defeated, 56-75. This game was part of the Ciutat de L'Hospitalet tournament, which led up to the Nike International Junior Tournament (NIJT). On January 5, he scored a personal best of 16 points on the Spars Sarajevo youth squad, helping Sevilla beat their opponents by a margin of 8 points. Porziņģis said, "I knew it was a prestigious tournament and that I had to do well", he said. "I think I could have done much better, but I wasn't physically 100 percent. I have seen videos and could have been much more aggressive. But I wish I could have played better." He finished the tournament averaging 9.2 points, 4.8 rebounds and 2.6 blocks. Porziņģis represented the Sevilla junior team again at the same tournament in early 2013. Against Union Olimpija on January 4, he recorded 15 points and 6 rebounds. On January 6, in the team's tournament finale vs Real Madrid's youth squad, he scored 24 points and grabbed 11 rebounds. He also made a total of five three-pointers. Porziņģis proved to be more effective in his second year, averaging 16.6 points, 8.4 rebounds and 2.6 blocks and going .481 on three-pointers. This would be his final stint in the youth categories of Sevilla.
Entering the 2012–13 ACB season, Cajasol Sevilla had hired Aíto García Reneses, who previously worked with the likes of Pau Gasol, Juan Carlos Navarro, Ricky Rubio and Rudy Fernández. On September 29, 2012, Porziņģis got the opportunity to make an appearance with the club's first team, but was allowed to play just a minute. Sevilla took a tremendous loss to CB Murcia. Following a return to the youth categories of the club at the Torneig de Bàsquet Junior Ciutat de L'Hospitalet, he went back to competing for the senior squad. On January 16, 2013, he logged only four minutes against Spartak Saint Petersburg in the EuroCup, scoring no points, but contributing one rebound and one assist. He scored his first basket for the senior team on February 20, 2013, in a rematch with Spartak. Porziņģis said, "I was very nervous at the beginning: I wanted to do well and not pick up turnovers. At the same time, I got a lot of confidence with players of my same age. That allowed me to play better with the first team and practice with more confidence." On May 4, 2013, he posted a season-high 7 points with the senior team, hitting two of three shots vs Bilbao Basket in ACB competition.
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 November 2, 2013. He notched a career-high 12 points, 6 rebounds, and 4 blocks. He broke his scoring record once more against Real Madrid on November 30, 2013, nearly leading his team to a victory. Against the very same team on April 6, 2014, Porziņģis scored a career-high 20 points, nailing 2 three-pointers. Despite his team losing, he began to trend on social media because of his performance. He said, "It was a very good game for me on offense. I didn't get any rebounds in that game. I had a good shooting night, but could have helped more on defense. Still, fans in Madrid gave me a nice ovation when I fouled out and I liked that a lot." On May 25, he got the chance to face his idol Justin Doellman of Barcelona, who he called the "best power forward in Spanish basketball". He scored 14 points against Doellman's team. On May 2014, Porziņģis was selected in the ACB All-Young Players Team of the 2013–14 season.
Prior to the 2014–15 ACB season, head coach Aíto García Reneses parted ways with the Sevilla. On October 4, 2014, against CB 1939 Canarias, Porziņģis made his season debut in the Liga ACB, scoring 3 points throughout the game. He made his first appearance at the 2014–15 EuroCup on October 15, vs. EWE Baskets Oldenburg and scored just 2 points. However, he had a strong EuroCup performance against Pallacanestro Virtus Roma in the weeks that followed, in which he contributed 18 points, 7 rebounds, 4 assists, 4 steals and 2 blocks. He recorded a double-double in mid-November in a win over Estudiantes, adding 11 points and 11 rebounds. On February 11, 2015, he scored a season-high 19 points against the EuroCup's Turów Zgorzelec. He tied this record on April 18, 2015, vs. Bàsquet Manresa in the ACB. On April 15, 2015, Porziņģis was named the winner of the EuroCup Rising Star Award of the season. One month later, he repeated in the ACB All-Young Players Team, after avoiding the relegation with Baloncesto Sevilla.
On April 28, 2014, Porziņģis declared himself eligible for the 2014 NBA draft. Before and after he made the decision, he 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. The Oklahoma City Thunder reportedly were certainly going to select him with the 21st pick in the case that he remained. Porziņģis was considered an unfinished product and a top-15 draft pick. According to DraftExpress.com, he was the fourth-youngest prospect in their top-100 rankings. Shortly before the day of the draft, however, he withdrew his name. 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. One of the league's general managers commented on him, "He's very talented. He wasn't ready, but we would've seriously considered drafting him anyway. If he continues to develop his game, get more minutes and his body develops, I think he could be a top-five pick in 2015. He has that kind of talent."
On April 16, 2015, Porziņģis entered the 2015 NBA draft, according to a report from agent Andy Miller. After gaining a season more of experience, he became known as a lottery pick and a potential top-5 selection. He drew interest from teams such as the Los Angeles Lakers, who had the No. 2 pick and had several personnel that had seen Porziņģis play in Spain. He was compared with players such as Pau Gasol and Dirk Nowitzki, but also Darko Miličić, a former lottery selection widely considered a draft bust. Adrian Wojnarowski of Yahoo! Sports wrote, "Porziņģis has an innate awareness about the way the American public sees a young, long European teenager. He comes to the NBA with the full understanding that popular basketball culture declares him guilty until proven innocent of the basketball crimes of Darko Miličić and Nikoloz Tskitishvili and Andrea Bargnani. He's considered a stiff, a bust, a blown lottery pick until he doesn't become one..."
On June 25, 2015, Porziņģis was selected with the fourth overall pick in the 2015 NBA draft by the New York Knicks. He was booed by the New York crowd upon being drafted, but vowed to change the fans' opinions on him from negative to positive. On the same night, the New York Knicks traded for Porziņģis' teammate from Sevilla, Willy Hernangómez, who was originally drafted by the Philadelphia 76ers as a 35th overall pick. Porziņģis became the highest drafted Latvian and Baltic player in NBA history.
On July 30, 2015, Porziņģis signed his rookie-scale contract with the Knicks. He made his debut for the Knicks in the team's season opener against the Milwaukee Bucks on October 28, recording 16 points and 5 rebounds in a 122–97 win. On November 2, he recorded his first career double-double with 13 points and 14 rebounds in a loss to the San Antonio Spurs. On November 21, he had a season-best game with 24 points, 14 rebounds and 7 blocks in a 107–102 win over the Houston Rockets. Over his first 18 games, he ranked third among all rookies in scoring (13.7 ppg) and was second in rebounding (9.3 rpg) and blocked shots (1.89 bpg) for October and November. He subsequently earned Eastern Conference Rookie of the Month honors for games played in October and November. On December 7, in a loss to the Dallas Mavericks, Porziņģis led a fourth-quarter charge and finished with a game-high 28 points, facing off against fellow European big man Dirk Nowitzki.
On January 4, 2016, he was named Eastern Conference Rookie of the Month for December. Porziņģis missed his first game of the season on January 28 after being diagnosed with an upper respiratory illness. On February 2, he was named Eastern Conference Rookie of the Month for January. From the months of October to January, he and Minnesota's Karl-Anthony Towns (Western Conference) both won all three monthly honors for the season. On February 12, he scored 30 points for Team World in the 2016 Rising Stars Challenge. On March 23, he tied a career high with 29 points and grabbed 10 rebounds in a 115–107 win over the Chicago Bulls. He appeared in 72 of the team's 82 games in 2015–16, missing the final seven games of the season due to a right shoulder strain. He finished with averages of 14.3 points, 7.3 rebounds, 1.3 assists and 1.9 blocks per game. Porziņģis finished second in the NBA Rookie of the Year Award voting behind winner Karl-Anthony Towns, and earned NBA All-Rookie First Team honors.
On November 4, 2016, Porziņģis scored a season-high 27 points in a 117–104 win over the Chicago Bulls. He topped that mark on November 16 with a career-high 35 points in a 105–102 win over the Detroit Pistons. On December 11, he recorded 26 points, 12 rebounds and a career high-tying seven blocks in a 118–112 win over the Los Angeles Lakers. On January 19, 2017, he 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. He subsequently scored 15 points in a 113–110 loss to the Washington Wizards. During the 2017 All-Star Weekend, Porziņģis played for Team World in the Rising Stars Challenge and won the Taco Bell Skills Challenge.
Porziņģis played with the Latvian youth team and was selected in the 2013 FIBA Europe Under-18 Championship All-Tournament Team.
In 2015, Porziņģis was named the Latvian Rising Star of the Year.
Porziņģis has been strongly compared to Pau Gasol, but has been even more praised than the NBA player in terms of athleticism. A general manager in the NBA commented on Porziņģis's 2015 NBA draft stock, saying, "I've been asking my team since then: Are you sure he's not the No. 1 guy? Are these guys in college really better than him?" Over the course of the 2015–16 season, he demonstrated his dominant offensive rebounding ability and showed signs of being a proficient scorer, as he became the first rookie in NBA history to record 1000+ points, 500+ rebounds, 75+ three-pointers and 100+ blocks.
Porziņģis was born to parents who had experience playing the game of basketball. Tālis, his father, competed semi-professionally before becoming a bus driver. His mother, Ingrīda, was previously on Latvia women's youth national basketball team. Kristaps's older brother Jānis, who is about thirteen years older than him, also played prfessionally, while Mārtiņš, who is approximately fifteen years older than his youngest sibling, was also an avid player.
Jānis Porziņģis competed at the European 2nd tier EuroCup level, the same level in Europe that Kristaps later played at, in one game, and played European professional club basketball in various national leagues, including the Italian League, for more than 10 years. He is known to mentor his younger brother on and off the court and often called him after playing games for Cajasol Sevilla in Spain. Kristaps talked about the relationship in an interview, "We'd break down the details. We watched the film together. He's always pushing me to work hard. We just spend a lot of time together and we just talk about basketball all the time..." The elder Porziņģis helped him train in the summer and work out in the gym in preparation for international competition in 2012.
Following two seasons in Spain and almost two years living in Seville, Porziņģis was able to speak Spanish far more fluently. Porziņģis is thus fluent in three different languages, including English. An executive said, "He speaks great English and I don't see it being that difficult of a transition off the court."
Porziņģis is a football (soccer) fan and supports his hometown team FK Liepāja, as well as Real Madrid CF and Sevilla FC.
In October 2016, Porziņģis signed a shoe deal with Adidas, the most lucrative deal for a European player. He made the switch to Adidas after partnering with Nike for his rookie season.