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.

He completed a fantastic tournament, scoring 16 points in the final game, and was named the Eurocamp's top small forward.

In the summer of 2013, he returned to Real Madrid, becoming a more standard part of the 12-men roster that won the Spanish League championship and finished second in the 2012–13 Euroleague.

On 19 May 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.

[6] On 5 July 2019, Díez signed a two-years deal with an option for a third season with Iberostar Tenerife of the Liga ACB.

[13] On 27 June 2024, Diez's rights were traded back to the Portland Trail Blazers as part of a package for Tyler Kolek.

Dani Díez continued to improve his game to become one of the most important players for Spain, which resulted in him being selected to the All-Tournament Team for the 3rd consecutive year.

[19] His averages improved significantly compared to previous years, to 18.7 points (top scorer of the tournament) and 8.8 rebounds per game.