The Harry Potter star fears many youngsters are becoming so used to shortening their words and sentences for the microblogging site, they are losing their ability to express themselves properly.
Fiennes is also adamant many will struggle to understand the classic texts of William Shakespeare.
He tells Britain's Evening Standard, ""We're in a world of truncated sentences, soundbites and Twitter. (Language) is being eroded - it's changing. Our expressiveness and our ease with some words is being diluted so that the sentence with more than one clause is a problem for us, and the word of more than two syllables is a problem for us.
""I hear it, too, from people at drama schools, who say the younger intake find the density of a Shakespeare text a challenge in a way that, perhaps, (students) a few generations ago maybe wouldn't have.
""I think we're living in a time when our ears are attuned to a flattened and truncated sense of our English language, so this always begs the question, is Shakespeare relevant? But I love this language we have and what it can do, and aside from that I think the themes in his plays are always relevant.""