The decision follows the company's earlier rejection of Monsanto's $45bn proposal last month.
Syngenta rejected the offer saying that its rival company had not seriously addressed the regulatory concerns over the potential deal.
In a statement Syngenta said: "Monsanto's second letter represents the same inadequate price, same inadequate regulatory undertakings to close, same regulatory risks, and same issues associated with dual headquarters moves. As such, we have reiterated our prior rejection of Monsanto's proposal."
"We remain committed to unlocking the opportunity of this combination and pursuing constructive conversation with Syngenta's management and board."
Monsanto has reportedly been trying to buy Syngenta in an effort to create a base for itself in Switzerland to benefit from lower taxes.
In addition, the takeover would give Monsanto access to a broad portfolio of fungicides, insecticides, and other herbicides.
Monsanto Chairman and CEO Hugh Grant said: "It is disappointing that Syngenta has not engaged in substantive discussions about the many benefits of this combination, including the benefits for farmers around the world. We remain committed to unlocking the opportunity of this combination and pursuing constructive conversation with Syngenta's management and board.
"Monsanto devoted significant time and resources analyzing the potential combination with Syngenta, and we are confident in our ability to obtain all necessary regulatory approvals. We've backed our confidence by agreeing to divest overlapping businesses and offered a $2bn reverse break-up fee to further demonstrate our commitment to this combination."
