The 31-year old replaced Victor Valdes as a second-half substitute in the friendly game and has spoken of his happiness at the result, as well as the return to form of David Villa
The game took place in Estadio Rommel Fernandez Gutierrez in Central America, and it was a comfortable victory for the 2010 World Cup winners, with Pedro, David Villa, Sergio Ramos and Markel Susaeta getting amongst the goals.
And although Panama are rated 46th in the Fifa world rankings, some 45 places below the Spaniards, Casillas was pleased with the display from his team-mates.
"It was great," he told reporters.
"The Panamanians celebrated their goal like they had won the World Cup, they really are wonderful people."
Villa played in his second game for his country since his return from a broken tibia, and was given the captain's armband for the occasion, before handing it over on the arrival of Casillas.
Meanwhile, Panama goalkeeper Jaime Penedo, explained that he was disappointed to concede five goals, but it was always going to be hard against a team of Spain's stature.
He said: "It was never going to be a picnic, was it?
"Spain have so many world-beaters and so many virtues that it was always going to be difficult to play against.
"It was an experience and I will keep working to improve my game."