Minority government likely
Socrates kept the options open in his victory speech, saying it was "too early to talk about" coalitions or pacts.
He said only after consultations with the president and other political parties in mid-October would "everybody know what the political solution will be".
Antonio Costa Pinto, a political scientist, said a minority government was most probable.
"They will most likely form a government on their own and at least in the first year there should be some stability, with the main opposition party obviously weakened by the election result," he said.
With 99 per cent of votes counted, the Socialists were on course to hold 96 seats in the new parliament, down from 121 seats in the 230-seat parliament during Socrates' first term.
The voter turnout was 61 per cent.
Economy in focus
Sunday's election was dominated by disagreements on how to deal with Portugal's economic crisis.
Socialists have advocated a series of big infrastructure projects, such as a high-speed train link to Spain and a new Lisbon airport to boost jobs and growth, while the opposition Social Democrats campaigned for vigorous public sector spending cuts.
Unemployment is 9.1 per cent and rising, the highest rate since the 1980s.
With a budget deficit set to reach 5.9 per cent of gross domestic product this year, spending cuts or tax hikes will likely be needed.