I'm sure this is a boy... but who knows they never really said

No Medusa Said clearly in the anime that Chrona was her daughter.

as far as i know Chrona's true gender was never confirmed.
I would like to believe Chrona is a girl though....

BikkuriShita said:
No Medusa Said clearly in the anime that Chrona was her daughter.

http://souleater.wikia.com/wiki/Crona
I've been re-watching the series recently and haven't seen anything too definitive one way or the other. Medusa always referred to Chrona as "my child"

The author has never said one way or the other and has said he never will. Crona's gender depends on whoever is translating the chapter or episode.