Kent was the only county found without otters in a survey of rivers across England carried out by the Environment Agency (EA) last year.
Since then at least two otters have been spotted, with holts on the Medway and Eden rivers, the EA said.
A survey on the Ribble in Lancashire showed a 44% increase since 2008.
Otter numbers fell as a result of toxic pesticides, which damaged their health and reduced their supplies of fish. They had almost disappeared from England by the 1970s.'Great success'
Improvements in water quality, along with legal protection, has helped their recovery.
"The recovery of otters from near-extinction shows how far we've come in controlling pollution and improving water quality," said the EA's national conservation manager, Alastair Driver.
"Rivers in England are the healthiest for over 20 years and otters, salmon and other wildlife are returning to many rivers for the first time since the Industrial Revolution.
"The fact that otters are now returning to Kent is the final piece in the jigsaw for otter recovery in England and is a symbol of great success for everybody involved in otter conservation."
The otter survey of England, which examined 3,327 river sites between July 2009 and March 2010, showed the number of places with evidence of otter life had increased tenfold in 30 years.
But recovery was slowest in the South East, with conservationists predicting otters may not be resident in Kent for another 10 years.
Their return was also a "fantastic reward" for efforts by the agency to improve water quality, said Mr Driver.