"We have received material from a small archive from a periodical called the Jules Verne Magazine, and in that small archive there were some manuscripts by the author Stieg Larsson that were never published," Magdalena Gram at KB told AFP.
The manuscripts, written around 1970 when Larsson was just 17, were "in the science fiction genre" and had been sent to magazines in hope of publication,
She said the library had been aware of the manuscripts for some time, but that she had only just started looking at the papers and could provide no further details.
Gram said it would be up to Larsson's estate-holders - his father and brother - whether to publish the works, but said they should think twice before doing so, since the early works could potentially harm the author's reputation.
Larsson's Millennium trilogy has become a phenomenon in Sweden and abroad, translated into more than 30 languages and made into popular Swedish movies with Hollywood versions in the works.
Its popularity is a striking contrast to the author's tragic fate.
Larsson, who worked as a journalist in Stockholm before writing the books, did not live to enjoy the sensational success; he died in November 2004 of a heart attack, aged 50, a year before the first book was published.