The broadcaster said it had reported the matter to the police.
On Thursday, Chief Executive Carrie Lam, Security Secretary John Lee and Police Commissioner Chris Tang were sent mail containing a similar powder.
The letter sent to Lam arrived on her birthday.
Media reports said the envelopes contained not only the mysterious powder, but also a letter that contained Chinese characters that made little sense.
No one was injured in the incidents.
Police classified the cases as “discovery of suspicious objects”, adding that the powder did not contain any dangerous substances.
Sources told RTHK that the powder was likely to be flour.