It is believed the 18 year old was struck in the head with a bottle during a disturbance in the Beechwood area of Lifford around 4am.
The man was rushed to Letterkenny General Hospital where his condition is described as “serious.”
Nobody has yet been arrested in relation to the attack.
However Gardai have appealed for anyone who may have been in the area and witnessed the disturbance to contact them.
“We are asking people who may have witnssed this attack or knojw anything about it to come forward.
“This is a serious matter which has resulted in a young man being hospitalised with serious injuries,” said a Garda source.