Díez started his playing career in 2005 in the U-14 Adecco Estudiantes, team that left to join the U-18 Real Madrid Baloncesto in 2009. The following season, 2009–10, he debuted for the Real Madrid Baloncesto B team in LEB Plata League. During the 2011–12 season he became one of the more important Real Madrid Baloncesto B players, coached by Alberto Angulo, and also began to train and travel with the Real Madrid first team. Díez finally debuted in the ACB League, in the match against Assignia Manresa played on December 18, 2011. In June 2012 he was invited to the Adidas Eurocamp in Treviso, one of Europe's best showcase for young players. He completed a fantastic tournament, scoring 16 points in the final game, and was named the Eurocamp's top small forward. During the 2012–13 season he played on loan for ACB League's Lagun Aro GBC. In the preseason he was one of the most outstanding players of his new team, averaging 10.2 points and 4 rebounds. In Week 5 of the Spanish League he had his personal best game with 17 points. In the summer of 2013, he returned to Real Madrid, become a more standard part of 12-men roster that won the Spanish League championship and finished second in 2012–13 Euroleague. On August 27, Díez started the preparation for the pre-season along with his teammates. On October, 5, they obtained the first title of the season, the Spanish Supercup, by defeating Barcelona in the final with 83–79. In September 2014, Díez was loaned again to Gipuzkoa Basket. He was named MVP of week 20 of the 2014–15 ACB season, after obtaining 32 points and 36 in PIR against Andorra. He has also been included in the best team of the 2014–15 ACB season in weeks 15, 20 and 31. On May 19, 2015, Díez was named the Best Young Player of the competition, after averaging in his second year with Gipuzkoa Basket 12.6 points and 7.0 rebounds per game. On 8 July 2015, Díez signed a two-year contract with Unicaja. In April 2017, Díez won the EuroCup with Unicaja after beating Valencia BC in the Finals. On July 5, 2019, Díez signed a two-years deal with an option for a third season with Iberostar Tenerife of the Liga ACB. He extended his contract until 2023 on July 3, 2020.
NBA rights
On June 25, 2015, he was selected with the 54th pick of the 2015 NBA draft by the Utah Jazz and his rights were afterwards traded to the Portland Trail Blazers for cash considerations.
In 2009, they won the gold medal with the Spain national U-16 team in the 2009 FIBA Europe Under-16 Championship. In July 2010 he played at the 2010 FIBA Under-17 World Championship, in Hamburg. He was the leading scorer and rebounder for the Spanish team, averaging 12.9 points and 7 rebounds per game. In the summer of 2011 he won the gold medal at the 2011 FIBA Europe Under-18 Championship, being selected in the All-Tournament Team. He averaged 12.2 points and 10.4 rebounds in 26.6 minutes per game over the competition. In July 2012 he played with the Spain national U-20 team and obtained the Bronze Medal in the 2012 FIBA Europe Under-20 Championship. Despite being only 19 years old, he was selected in the All-Tournament Team, averaging 14.3 points and 5.9 rebounds per game, with a 56% field goal percentage. Díez was the leading scorer and rebounder for the Spanish team, playing both the forward and power forward positions. In the summer of 2013 he won again the Bronze Medal in the 2013 FIBA Europe Under-20 Championship. Dani Díez continued to improve his game, being the most important player of the Spanish team and member of the All-Tournament Team for the 3rd consecutive year. His averages improved significantly compared to previous years, to 18.7 points and 8.8 rebounds per game. Dani Díez has played 7 times with the Spain men's national basketball team.
Individual awards
2011 FIBA Europe Under-18 Championship: All Tournament Team
2012 FIBA Europe Under-20 Championship: All Tournament Team
2013 FIBA Europe Under-20 Championship: All Tournament Team
