Boat hire firm Blue Funnel Cruises has agreed to buy Hythe ferry, which crosses Southampton Water, for an undisclosed sum.
The deal includes the purchase of Hythe Pier and the railway that runs along its 640m (2,000ft) length.
The ferry's previous owners warned in October of declining passenger numbers and possible job losses.
More than 9,000 people signed a petition to save the 19th Century pier and ferry.
They won support from television historian Dan Snow, who said the ferry and "the oldest running pier train anywhere in the world" were "national treasures".
Blue Funnel Cruises said it would introduce a second, smaller vessel in two months' time, to improve reliability and attract more passengers.
It said the ferry service lost "a lot of custom" after a previous vessel was damaged when it hit the pier in May 2016.
The crash was caused by mechanical failure, investigators found.
Blue Funnel director Lee Rayment said the "cold and uncomfortable" vessel would be upgraded, with heating and new seats instead of wooden benches.
He said there were no plans for redundancies or increased fares.
Mr Rayment said the pier could eventually be sold or leased to a community group to restore it.
Peter King from the Hythe Pier Train and Ferry Action Group said the pier and ferry service's immediate future had been secured.
He said the group would hold discussions with Blue Funnel about the pier, which he said needed "substantial long-term investment".
Mr King said a redevelopment project could cost £2m to £3m but a trust would be able to access other sources of finance, including lottery grants.
A ferry service that had faced an uncertain future is to be run by a new operator from Friday.