Steve Feltham had hoped to complete his task in three years after giving up his job and selling his home in Dorset to move to Dores near Inverness.
Tuesday marks the 25th anniversary of his effort to explain the monster.
He said: "In that time I had one sighting of something I could not explain, but I can't say what it was."
The 53-year-old is recognised by the Guinness Book of Records for the longest continuous monster hunting vigil of Loch Ness.
Mr Feltham, who began his search in 1991 and has been fascinated by the mystery since childhood, said: "I look at 25 years as a good halfway mark in trying to solve this.
"I am willing to dedicate another 25 years. Hopefully it will only take another few weeks. Who knows?
"When I first came here I had no clear idea how long I would need. But I thought I would see something in three years."
May this year also saw the marking of 20 years of the Official Loch Ness Monster Sightings Register.
Inverness-based chartered accountant Gary Campbell, the keeper of the register, has logged more than 1,000 sightings since the record started.
Mr Campbell, who believes a large fish or eel is behind the monster claims, said the majority of sightings were not included because they could be explained.
A man who dedicated his life to solving the mystery of the Loch Ness Monster 25 years ago says he is prepared to commit another 25 to his search.