The deal means that Syngenta's flower seed division will no longer be sold off.
Syngenta chairman Michel Demaré said: "ChemChina has a very ambitious vision of the industry in the future. Obviously it is very interested in securing food supply for 1.5 billion people and as a result knows that only technology can get them there."
The company had a $46bn takeover bid from US rival Monsanto in 2015 rejected on competition grounds.
Syngenta chief executive John Ramsay said: "Syngenta is the world leader in crop protection. This deal will enable us to maintain and expand this position, while at the same time significantly increasing the potential for our seeds business."
ChemChina chairman Ren Jianxin said talks had been "friendly, constructive and co-operative".
He will become chairman of the Swiss company and four of its existing directors will be on the 10-member board.
The Chinese company also owns Pirelli, KarussMaffei and Israeli pesticides company.
In December 2015, Dow/DuPont merged in a $130bn move.
Ramsay added: "Syngenta is the world leader in crop protection having significantly increased its global market share over the last ten years. This deal will enable us to maintain and expand this position, while at the same time significantly increasing the potential for our seeds business. It will ensure continuing choice for growers and ongoing R&D investment across technology platforms and across crops. Our commitment to cost and capital efficiency will remain unchanged."
Jianxin added: "The discussions between our two companies have been friendly, constructive and co-operative, and we are delighted that this collaboration has led to the agreement announced today. We will continue to work alongside the management and employees of Syngenta to maintain the company’s leading competitive edge in the global agricultural technology field."
He added: "Our vision is not confined to our mutual interests, but will also respond to and maximize the interests of farmers and consumers around the world. We look forward to Michel Demaré remaining on the Board as Vice Chairman and lead independent director, and to working with John Ramsay and the management and employees of Syngenta to deliver safe and reliable solutions for the continued growth in global food demand."