He became his country's first ever medallist at the World Junior Championships in Barcelona, missing his aim of the championship record but still winning by a margin of more than half a second in 44.85. Acknowledging his quick progression, he said: "so far only Americans have finished in under 44 seconds, I want to be the first from somewhere else". He was beaten to that distinction by Grenada's Kirani James in the 400 m final at the 2012 Summer Olympics. Santos was next to finish after James, however, as he ran within 0.01 of his personal best time to claim an unexpected Olympic silver medal. His was the third ever medal for the Dominican Republic, coming just 45 minutes after Felix Sánchez won his second Olympic gold. A team of Gustavo Cuesta, Felix Sánchez, Joel Mejia and Santos appeared to have qualified for the 4 × 400 m relay final, but the second baton change was outside of the changeover zone and the team was disqualified.