On 28 November 2021, he replaced Franco Varrella as manager of the senior national team.
He made his debut on the bench of the Titans on 25 March 2022 against Lithuania in a friendly, suffering a defeat (1-2 for the Baltic national team).
In November 2021 following the departure of Franco Varrella, Costantini was named manager of the San Marino national team.
He was coming off of a four-year stint as manager of San Marino's national under-21 side.
[3] On 12 December 2023, the San Marino Federation announced in a statement that it had not renewed Costantini's contract, who therefore concluded his experience at the helm of the Titans with two draws (both obtained in the autumn of 2022, in as many friendly matches against the national teams of Seychelles and Saint Lucia) and 17 defeats out of 19 official matches.