Spain's fifth prime minister since the restoration of democracy after the 1975 death of General Franco, Zapatero was administered the oath of office on Saturday by King Juan Carlos.
Shortly after, the new prime minister named several political heavyweights in his new 16-member cabinet. Magistrate Jose Antonio Alonso was made the new interior minister, tasked to fight terrorism.
One of Zapatero's first acts as the prime minister was to visit survivors of the 11 March train bombings in Madrid in hospitals and to lay a bouquet of red roses at Atocha railway station, the main station hit by the blasts.
His swearing-in followed Friday's parliamentary vote, where the 350-seat chamber gave Zapatero 183 votes to 148 against and 19 abstentions, to allow him the absolute majority which his Social Party had missed by 12 seats in the 14 March election.
To make the shortfall, 19 non-Socialist deputies voted for the 43-year old career lawyer, who shook hands cordially in his oath taking ceremony with outgoing conservative Prime Minister Aznar.
Aznar's dogged pro-US stance over Iraq, coupled with his early insistence that Basque separatists and not Islamist extremists carried out the Madrid bombings which killed 191 people, helped swing the election to the Socialists.
Zapatero has promised a clean break with the Aznar years, pledging to withdraw Spanish troops from Iraq unless the United Nations took charge.
Apart from overhauling foreign policy, the new prime minister has promised to mould a new society, among other things backing gay marriages.