The offer of US$122 per share represents a 37% premium to Monsanto's share price before rumours of a bid emerged.
According to Reuters, Bayer has offered to buy Monsanto for US$62 billion in cash, including debt.
The acquisition would create the world's largest farm supplier eclipsing a planned combination of Dow Chemical and DuPont's agriculture units. It follows Werner Baumann becoming Bayer CEO.
The offer of US$122 per share represents a 37% premium to Monsanto's share price before rumours of a bid emerged.
"We fully expect a positive answer of the Monsanto board of directors," Baumann told reporters.
Monsanto has yet to comment.