* Former post-independence communist leader wins vote
* Pinto da Costa pledges to fight corruption, poverty
* Concerns over slide back to authoritarian rule -analyst (Adds details, analyst comment)
By Ricardo Neto
Sao Tome, Aug 8 (Reuters) - Manuel Pinto da Costa has won a presidential election runoff in Sao Tome and Principe, the election commission confirmed on Monday, returning to power again in the tiny Gulf of Guinea island state after two decades.
Pinto da Costa, a 74-year-old economist by training, ruled the former Portuguese colony as a single-party state from independence in 1975 until 1991.
Sao Tome's national elections commission said Pinto da Costa won Sunday's second round vote with 52.88 percent against his opponent, parliament speaker Evaristo Carvalho.
IHS Africa analyst Sebastian Boe said Pinto da Costa's victory signalled the frustrations of the country's voters with the graft and mismanagement plaguing the country.
He said the election also raised concerns that the country might slide back to the authoritarian rule seen during Pinto da Costa's previous period in power.
"Many observers fear that his election is a sign of an imminent regression back towards the one-party state he once ran, possibly with grim implications for the state of democracy in the country," Boe said in a note.
Pinto da Costa said during the campaign that his priority would be to find ways to improve the Central African nation's economy after the oil sector failed to produce any major finds during several years of exploration.
The newly elected president told Reuters after the provisional results were announced on Sunday that his other priority would be to fight corruption and poverty in the Atlantic archipelago of over 140,000 people.
The government is keen to develop tourism in Sao Tome, which boasts largely unspoilt, palm-fringed beaches. It also has a small cocoa sector with a reputation for high-quality beans.
Writing by Bate Felix; editing by David Lewis and Gareth Jones