It scooped a silver award in the coastal category and won the Public Park Award for Ravelin Park in Old Portsmouth.
Southsea was one of 69 areas competing for awards judged during the summer by experts from the Royal Horticultural Society, who travelled the length and breadth of the UK to scrutinise lawns and landscaping.
Judge Jeff Bates said: 'Southsea was extremely colourful and very attractive with lovely residential gardens and wonderful natural beaches.
'We were especially impressed with Ravelin Park and pushed for it to win its award. The park is beautiful with some very special features including the community orchard and meadow – both very unusual for a city park.'
Ravelin Park manager Tony Davis was delighted to pick up the national award.
He said: 'It's absolutely superb, it's worth all the hard work we have put into the park.
'Eight years ago, Ravelin Park had hardly any maintenance at all. We've transformed it and hope to go from strength to strength.'
Southsea picked up the awards after gaining eligibility for the national finals following on from success in last year's South and South-East in Bloom.
Mr Bates was impressed with many aspects of Southsea, ranging from the lack of litter and dog fouling to colourful residential gardens and lush floral bedding around the seafront areas.
He also praised the nature reserve at St Jude's School, calling it a 'wonderful resource'.
Teacher Jackie Peters set up the 'Forest School' three years ago to aid the learning of four and five year olds.
She said: 'It's a fantastic learning environment and the kids are out there in all weather.
'I'm really glad the judges saw it and praised the area – it's great.'
Portsmouth City Council cabinet member for culture Cllr Terry Hall, said: 'I'm delighted that we have won a silver award, and I'm sure we will continue to impress the judges in years to come with a city which is both beautiful and sustainable.'