On 16 March 2018, Alonso received his first call-up for the Spain national team for friendlies against Germany and Argentina later that month. He debuted against the latter on the 27th in a 6–1 win at the Wanda Metropolitano where he replaced Jordi Alba with 11 minutes left, making the Alonso’s the inaugural Spanish family to have three generations of internationals and seventh worldwide. Alonso’s grandfather, Marcos Alonso Imaz, played with Real Madrid’s first team for eight years. His father, Marcos Alonso Peña, played several seasons in Spain’s top flight, most notably for Atlético Madrid and Barcelona, and both represented Spain at senior level. On 3 May 2011, Alonso was arrested following his involvement in a car accident in Madrid. He was the driver of a car that collided with a wall, killing one of the passengers, a 22-year-old woman, having been driving at 112.8 km/h (70 mph) in wet conditions in a 50 km/h (30 mph) zone, with a blood alcohol level of 0.93 mg per millilitre of blood. He faced 21 months in prison when sentenced in February 2016, but his punishment was changed to a €61,000 fine and a driving ban of three years and four months, which had already been spent.