MADRID — Adolfo Suárez, Spain’s first elected prime minister after the Franco dictatorship and a key figure in the country’s transition back to democracy, died here on Sunday. He was 81.

A family spokesman, Fermín Urbiola, announced the death. Mr. Suárez was admitted to a Madrid hospital last Monday with a respiratory infection that developed into pneumonia. He had been treated for Alzheimer’s disease for a decade.

A lawyer by training, Mr. Suárez led a new generation of Spanish politicians who filled the power vacuum left by the death of Gen. Francisco Franco in late 1975....