One man was in the water by the time the lifesavers arrived, with the craft getting into difficulties around one mile off Farlington Marshes in Langstone Harbour.
A dramatic video shows a rescue helicopter and a lifeboat crew at the scene of a man who got into difficulty after rescuers sped to the incident.
The volunteer crew was called out at 7.35 pm following a 999 call to the Coastguard reporting a small sailing dinghy in trouble.
Portsmouth RNLI said “Whilst approaching the scene, it was advised that the dinghy had got into difficulty and the person ended up in the water.
We were soon on the scene however due to low waters were unable to gain access to the person in difficulty the person was however able to wade through waters to safety.
The man was then met by rescue crews onshore along with a crew from South Central Ambulance Service, who arrived to check the man over.
Lifeboat crews then recovered the dinghy after the un-maned dinghy floated off in waters.
The spokesman added: “We would always recommend that you check the weather conditions prior to going afloat, particularly on the sea.
“If you do get into difficulty, please call for assistance as soon as possible. There is sea safety and advice for various watersports available on the RNLI website.”
Portsmouth RNLI is staffed by volunteers who regularly attend emergencies in the Forth after people get into trouble.
One local congratulated the team, saying: “Could have been a different outcome if it wasn’t for you guys, well done, be proud.”
Another added: “You guys are totally amazing. Thank you for all you do.”
A third person said: “Professional job done as always.”