The family-owned Park Avenue Hotel was awarded the extra star by the Northern Ireland Tourist Board, according to the Press Association.
Mandy Martin, director of the 56-bedroom hotel and grand-daughter of facility founder Harry Stephens, said the accolade reflected the hard work of Park Avenue's 100 employees.
She added that it was "unique" as a family-run hotel with strong local ties.
"But the business is also central to the bigger tourism picture providing east Belfast with a quality boutique-style hotel which attracts an enviable occupancy level," she told the news agency.
Ms Martin said the recent £5 million investment was an expression of its confidence in the Northern Ireland tourism sector.
In other Belfast venues news, the strength of the euro against the pound has been a boost to tourism in the city, helping it to attract record numbers of visitors, according to Belfast City Council figures.