"There is no excuse for Rossi's reaction and it represents a complete disgrace to everyone making a living from football," the FSS said on its website on Thursday.
"We do not approve of Ljajic's gesture either because players always have to accept a coach's decision and he should have done so in a more professional manner."
Fiorentina sacked Rossi on Thursday and named the club's administrative manager Vincenzo Guerini, whose previous role included duties such as organising kits and buses, as his replacement.
The incident occurred when Ljajic sarcastically applauded the coach when he was substituted in the first half with Fiorentina trailing their relegated Serie A rivals 2-0.
Rossi then lost his cool, grabbed Ljajic and landed two blows before he was restrained by substitutes and other club officials in the dugout.
The FSS applauded Fiorentina's decision to dismiss Rossi as coach.
"Fiorentina took the right course of action by relieving Rossi of his post because what he did is completely alien to sport and the fair-play code," said the Serbian FA.
"We wish to maintain our sound relations with Fiorentina but most of all we would like to extend our unreserved support to Ljajic in what must be a very difficult moment in his career."
Ljajic's career seems to be stuttering, with the 20-year-old failing to live up to the potential which prompted his 2010 move to Serie A from Serbian champions Partizan Belgrade.
Despite occasional flashes of brilliance, the attacking midfielder also struggled at Fiorentina under compatriot Sinisa Mihajlovic who made way for Rossi in November and is now set to take over as Serbia coach from caretaker Radovan Curcic.
With Fiorentina taking disciplinary action against Ljajic, his flailing international career also seems to be hanging in the balance with former Yugoslavia midfielder-turned-defender Mihajlovic known for his no-nonsense approach.
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