The agency won the brief after a competitive pitch that began last November.
Big Al's Creative Emporium will handle the launch of a new Roundup product across Europe next year, focusing on key markets including the UK, France and Germany.
Previously, Roundup has used Flint Creative for its TV advertising in the UK. The agency had held the business since 2002 when the brand began advertising on TV.
In 2009, Monsanto appointed the Birmingham-based HRO'C Digital as its first lead digital agency for Roundup, as the brand moved away from focusing on above-the-line work.
Gary Philpotts, the business director for Monsanto UK, said: "Big Al's demonstrated a creatively clear and enthusiastic approach and we feel we have a great partner."