Goals from Colby Bishop – one of which was a penalty – either side of half-time secured the Blues their first win of the new League One season.
It maintains Danny Cowley’s side’s unbeaten start to the season and moves Pompey up to ninth.
Here’s how The News rated the performance as temperatures rose to 33 degrees in Gloucestershire.
9. Michael Jacobs - 8
(Replaced by Ronan Curtis on 70 minutes): A busy performance which made him a constant threat . Photo: NationalWorld
10. Colby Bishop - 9
(Replaced by Jay Mingi on 88 minutes): Was red hot on a blistering warm day at Whaddon Road. Is proving one hell of a signing. Photo: NationalWorld
11. Dane Scarlett - 8
(Replaced by Joe Pigott on 67 minutes): Proved a constant thorn in the side. A real livewire who the Cheltenham defence will be glad to see the back of. Photo: NationalWorld
12. Joe Pigott - 6
(Replaced Dane Scarlett on 67 minutes): Some nice touches and looked lively to give hosts no respite in Scarlett's absence. Photo: Nationalworld