There is more than one way to determine which is the longest book of the Bible. For example, should we count the verses or the words? Some verses are significantly longer than others, so it is likely more accurate to count the number of words.
But, if we are counting the number of words, do we count the English words or the Hebrew/Greek words? Since the inspired original documents of the Bible were in Hebrew and Greek (see 2 Timothy 3:16–17), and since sometimes more than one English word is needed to translate a word in Hebrew or Greek (or sometimes the English requires fewer words than the original), a count using the number of Hebrew or Greek words would seem to be the most accurate.
By counting the number of words in Hebrew or Greek, the longest book of the Bible is Jeremiah, weighing in at 33,002 words. The second longest book of the Bible is Genesis with 32,046 words. While it is by far the longest book of the Bible by the number of chapters, the book of Psalms is only the third longest book of the Bible according to the number of words: 30,147. The fourth longest book of the Bible is Ezekiel with 29,918 words. The fifth longest book of the Bible is Exodus with 25,957 Hebrew words. The longest book of the New Testament, and the twelfth longest book of the entire Bible, is the Gospel of Luke with 19,482 Greek words.
Special thanks go to Logos Bible Software for making determining the longest book of the Bible by Hebrew or Greek word count much easier.