What manuscripts were used when translating the NIV?
The translators of the NIV have used the Old Testament and New Testament texts that are widely accepted among modern scholars as giving us the best possible access to what God inspired in the original documents.
For the Old Testament the standard Hebrew text, the Masoretic Text, as published in the latest edition of Biblia Hebraica, has been used throughout. The NIV translators have sometimes used variants of the Hebrew Masoretic tradition or other ancient versions, where these seemed to provide a superior text than the Masoretic tradition. These are all noted in footnotes.
The translators have used the accepted Greek New Testament text, as printed in the Nestle-Aland and United Bible Societies’ Greek New Testaments. Here also the translators have occasionally accepted a variant printed in these editions. Footnotes usually indicate the options in each case.
See more on this topic from the NIV Preface: