The actor, who stars alongside Fiennes in Wrath of the Titans, told The Daily Telegraph that Fiennes has in turn often been mistaken for him.
Neeson said: "I've been congratulated for my Hamlet on Broadway 14 years ago. And [Ralph] gets mistaken for me sometimes. We'd pass as brothers, you know.
"I was once down in Patagonia, Argentina, about five years ago fly-fishing, and this woman came up and said, 'Oh my God, Red Dragon! You were fantastic in that.
"I didn't want to say, 'It's not me', so I said, 'Thanks very much'."
Neeson also admitted to having once signed an autograph as Fiennes, recalling: "Once, my wife Natasha Richardson and I were walking through the Plaza Athenee, this beautiful hotel in Paris, and this elderly couple got up and said, 'Excuse me, my husband and I are visiting from America'.
''The lady grabbed one of these beautiful tea service menus and said, 'Would you mind signing this for us?'
"I said, 'It would be my absolute pleasure'. And as I'm about to sign it, she said, 'We absolutely adored The English Patient'.
"So I signed it, 'With love, Ralph Fiennes'. I could not burst their bubble.''
Neeson will next be seen in sci-fi action film Battleship alongside Taylor Kitsch, Alexander Skarsgård, Brooklyn Decker and Rihanna. A new clip from the film was released earlier today.
Battleship opens on April 11 in the UK, and on May 18 in the US.
Watch a trailer for the film below: