In comments to reporters on Tuesday evening, the presidential spokesman said that Bolsonaro was being neither inappropriate nor discourteous in calling Thunberg a "pirralha".
"It's incredible how much space the press gives this kind of pirralha", Bolsonaro added, using a Portuguese word that loosely translates as "little brat" or "pest".
"Indigenous people are being literally murdered for trying to protect the forrest (sic) from illegal deforestation", Greta Thunberg has said drawing the anger of the Brazilian head of state.
She had sailed to the Americas in the hope of attending the climate summit in Chile, but it was moved to Spain after protests broke out in the South American nation.
Bolsonaro had initially failed to remember Thunberg's name, labelling her simply "that girl". "So nice to see!"
"He should not worry about fighting Greta because she showed solidarity with the indigenous that were murdered", Silva said.
The strong words were a response to Greta Thunberg's condemnation of what the Swedish teenager described as the slayings of the Amazon natives.
The Guajarara tribe, who work to protect the rainforest, has been threatened by loggers who experts say have taken advantage of reduced controls and less oversight to seize control of forest lands.
"The enormity of the deforestation growth in Brazil is no accident", Christian Poirier, program director of the non-profit Amazon Watch, previously told Business Insider.