Sefer haYashar (Biblical references), Hebrew ספר הישר (also transliterated Sēper haiYāšār), 'Book of the Upright', often only half-translated into English as Book of Jasher or as Book of Jashar.

Joshua 10:13 states:

And then Sun stood still, and the Moon stayed,

until the people had avenged themselves on their enemies.

Is this not written in the Book of the Upright (Sēper haiYāšār)?

2 Samuel 1:18 states:

To teach the Sons of Judah the use of the bow; behold it is written in the Book of the Upright (Sēper haiYāšār). David's lament for Jonathan immediately follows.

The Septuagint translation renders sefer hayashar in both cases as 'Book of the Just'. The reference to the bow is here missing so that the text reads:

And he gave orders to teach it the sons of Iouda: behold it is written in the Book of the Just.

The Vulgate Latin text has in libro justorum in both citations. In 2 Samuel the Syriac texts have Ashir which indicates a Hebrew reading of ha-shîr 'the song' and in Joshua the Syriac has "Book of Praises".

The Targum on both passages has "Book of the Law". Yet the King James English translation rendered haiyashar as Jasher, removing the definite article and changing the second vowel. Some more recent translations render instead Jashar which at least restores the correct a vowel.

Since references in both books seem to be to poetic material, a common guess is that this Book of the Upright was a lost collection of old Hebrew verse.

Two works sometimes claimed to be this work are Sefer haYashar (midrash) and Book of Jasher (Pseudo-Jasher).

For other works of this name see Sefer haYashar.