Syngenta said in a statement the two companies had asked for the extension to allow "sufficient time for the discussion of remedy proposals".
The European Commission opened an in-depth investigation into state-owned ChemChina's $43 billion bid in October, saying the companies had not allayed concerns over the deal.
The Commission's website showed the deadline had been extended by 10 days on Tuesday.
"ChemChina and Syngenta remain fully committed to the transaction and are confident of its closure," the Swiss company said.
(Reporting by Julia Fioretti and Joshua Franklin in Zurich; editing by Susan Thomas)
