Please tell me whether kratom tea is halal or haram.

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger. 

One of the well-established Fiqh rules is that all things are halaal (lawful) in principle, unless proven otherwise by Sharia evidence, based on the verse: {It is He Who created for you all of that which is on the earth. Then He directed Himself to the heaven, (His being above all creation), and made them seven heavens, and He is Knowing of all things.} [Quran 2:29]

Shaykh As-Sa‘di wrote, "It means that He created for you, out of His kindness and mercy, all of that which is on the earth from which you benefit and derive enjoyment and lessons. This verse proves that the basic principle is that all things are pure and lawful because the context of the verse is reminding people of the bounties and blessings of Allah...”

If this herb does not cause what would make it haraam (unlawful) for human consumption, such as being a sort of drug that intoxicates, and if using it does not cause an outweighing harm or a harm that is equal to the benefit, then using it is permissible, Allah willing. However, if it is used like drugs or using it causes an outweighing harm or a harm equal to the benefit, then it is haram to consume it. The Prophet, sallallahu ‘alayhi wa sallam, said, “Harm may neither be inflicted nor reciprocated.” Moreover, warding off harm takes precedence over realizing a benefit.

Allah knows best.