The guarantees, which include financial commitments, form an essential part of the candidature file that must be submitted to the International Olympic Committee by October 15.
Gerardo Werthein, president of the Argentine Olympic Committee, said: "This is a major milestone for the Buenos Aires 2018 Youth Olympic Games bid. The fact that president Kirchner has signed the required head of state guarantees ahead of the submission of our candidate file to the IOC next month demonstrates the significant political support and commitment that exists for the Buenos Aires bid."
Mauricio Macri, the head of government of Buenos Aires City, added: "We have received support from the national government as part of the process all the Buenos Aires partners have undertaken to put together what we hope will prove a compelling candidate file to host the YOG 2018.
"We look forward to submitting our candidate file mid-October and demonstrating how we can host wonderful games and help the Olympic Movement connect with millions of new, young hearts and minds in Argentina and across South America."
The Argentinian capital faces competition from Medellin (Colombia), Poznan (Poland), Guadalajara (Mexico), Rotterdam (Netherlands) and Glasgow (Scotland) to stage the 2018 games.
The IOC executive board will draw up a shortlist in February 2013, after which an IOC Working Group will publish a report on the candidates.
IOC members will select the host city at their meeting in Lausanne on July 4.