Saltdean Lido was in a dilapidated state and in danger of being partially demolished until a local group secured a lease from the council to operate it.
The refurbished outdoor site now has a 40m (131ft) heated swimming pool, a paddling pool and a "splash area".
A synchronised swim marked the ticketed opening of the pool to the general public on Saturday morning.
Rebecca Crook, chairwoman and director of the Saltdean Lido Community Interest Company (CIC), said the facilities also included a new changing block, but there was still more work to be done.
"Our next phase subject to funding will see the main building restored with building works commencing in 2018," she said.
"This project demonstrates what a group of volunteers can do.
"We are delighted and hope many more future generations will enjoy the iconic building and pool."
The pools will be open until mid-September, but the aim is to eventually offer a range of all-year round amenities.
The Saltdean Lido CIC has a 60-year lease to operate the pool.
It was first opened on the seafront in July 1938 and was designed to face south-east towards the sea.
It was widely admired for its innovative design and the aim to give swimmers the impression of being on a cruise liner.
After just two summer seasons the lido was closed in September 1940 by the outbreak of World War Two.
It did not reopen until July 1964, by which time it had been bought and restored by Brighton Town Council for Â£20,000 after it issued a compulsory purchase order on the owners.
It closed again in 1995 after falling attendances and the need for repairs.
The Save Saltdean Lido Campaign was then set up in March 2010 by local residents after plans were announced to fill in the swimming pool, develop the site extensively and create 125 apartments.
Campaigners finally won their fight to run and restore the lido three-and-a-half years later.
A Grade II*-listed lido has reopened following a seven-year restoration project and Â£3m of community funding.