Question
Is it impermissible for the pilgrim, whether male or female, to wear underwear during Hajj?
Fatwa
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 is His Slave and Messenger.
The Prophet clarified and explained what men and women are allowed to wear in the state of Ihraam. It was narrated on the authority of Ibn ‘Umar, may Allah be pleased with him, that a man asked the Prophet about the clothes worn by the pilgrim in the state of Ihraam. The Prophet replied: “A Muslim [man] in the state of Ihraam should not wear the Qamees [shirt], a turban, a hooded cloak, or trousers, or a garment scented with Wurs or Saffron [types of perfumes], or [wear] leather socks except if he does not find shoes; he may [then] put on leather socks but he should trim them to be below the ankles.” [Al-Bukhari and Muslim]
It was narrated on the authority of Ibn ‘Umar, may Allah be pleased with him, that the Prophet said: “A woman in the state of Ihraam should neither wear the Niqaab nor wear gloves.” [Al-Bukhari] These texts clearly indicate the rulings upon which the scholars have unanimously agreed; that a man in the state of Ihraam should not wear form-fitting clothes on any part of his body, and that the woman may wear whatever she likes except form-fitting items on her face or her hands.
As for your question, underwear is a type of form-fitting clothing, so men are not allowed to wear them, whereas women are allowed. If the underwear is not form-fitting, such as wrapping the body with another piece of cloth under the upper or lower garment, then it is permissible for men to wear it. This is because the Muslim in the state of Ihraam is forbidden to wear form-fitting clothes, such as the Qamees, hooded-cloak, trousers, which also includes swimming suits, and underwear, as stated in the Hadeeth above.
And Allah knows best.