User blog comment:Benols/Is "Harry Potter and the Cursed Child" canon?/@comment-761097-20161127192312

Absolutely. Though not directly written by JKR it was okayed by her and therefore is canon. Other examples exist in fandom, for example, the Star Wars novels and comics, they were not written by George Lucas but were uthorized by LucasFilms and thus are canon. The same with the Star Trek franchise or Indiana Jones or Dr Who. Many different writers but all authorized by their respective franchises.