Steven Kabuye was "stabbed to near death by unknown assailants," his organization, Coloured Voices Media Foundation-Truth, said on the social media platform X.
A police spokesman said he was unaware of the attack and would look into it.
Sexual minorities say they have faced a wave of abuse since a harsh anti-gay law was enacted last year.
The law prescribes the death penalty for certain same-sex acts.
At least seven people have been arrested under it, and rights activists say it has also led to a surge of abuse by private individuals against LGBT people, including rapes, torture and beatings.
Uganda's government has denied this is happening and said the law is meant to criminalize same-sex activity and its promotion, not penalize LGBT Ugandans.
A Ugandan court heard a petition last month that says the law violates constitutionally protected rights.
It has not yet issued its ruling.
