Locsin, currently the ambassador to Britain and Ireland, often used strong language while serving as foreign secretary under former president Rodrigo Duterte and once swore at China online over the presence of its vessels in the disputed South China Sea.
"China, my friend, how politely can I put it? Let me see... O... GET THE FUCK OUT," Locsin posted on Twitter in May 2021.
Locsin also likened China to "an ugly oaf forcing your attentions on a handsome guy who wants to be a friend".
His expletive-laden demand prompted a rebuke from Beijing and Locsin later apologised to his Chinese counterpart.
The Philippine foreign affairs department declined to comment on Locsin's surprise appointment, which was announced by the Presidential Communications Office on its official Facebook page.
It said in a brief statement Locsin had been appointed "Special Envoy of the President to the People's Republic of China for Special Concerns".
No other details were provided.
Presidential communications chief Cheloy Garafil told reporters Locsin would serve in a "concurrent capacity", suggesting he would remain as ambassador.
Locsin is a prolific poster on the platform X, formerly known as Twitter, on topics ranging from Holocaust victims to his late pet cat but has not commented publicly on his appointment.
The decision comes at a delicate time for relations between the Philippines and China, which are embroiled in another diplomatic spat over the South China Sea.
Beijing claims almost all of the waterway, through which trillions of dollars in trade passes annually, and has ignored a 2016 international court ruling that its assertion has no legal basis.
Tensions flared this month when the Philippines accused Chinese Coast Guard vessels of blocking and firing water cannon at boats on a resupply mission.
Manila summoned Beijing's envoy over the incident, which resulted in one of the boats carrying supplies failing to reach a Philippine Navy vessel grounded on Second Thomas Shoal in the Spratly Islands.
The handful of Filipino marines stationed on the crumbling ship to assert the Philippines' territorial claims depend upon resupply missions to survive their remote posting.
Beijing has defended its actions as "professional", and accused Manila of "illegal delivery of construction materials" to the grounded ship.
The Philippines has insisted that Second Thomas Shoal is within its exclusive economic zone and its efforts to resupply troops and repair the BRP Sierra Madre are legitimate.
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